頂点を1点または2点選択して，その頂点の座標が指定した値の位置になるよう，選択したオブジェクト全てを拡縮する(実行は編集モードのメッシュから)　特定の箇所の大きさを基準にしてスケールを合わせたいときに

scaling_objects_by_selected_vert_location.py

bl_info = {
    "name": "Scaling objects by selected vert location",
    "author": "mol",
    "version": (1, 0),
    "blender": (2, 81, 0),
    "location": "View3D > Mesh",
    "description": "Scaling selected objects by specifying selected vert location",
    "warning": "",
    "support": "COMMUNITY",
    "wiki_url": "",
    "category": "Scaling Object",
}


import bpy
from bpy.props import FloatProperty
from bpy.props import BoolVectorProperty
from bpy.props import EnumProperty
from mathutils import Vector
import math


def axis_list_fn(scene, context):
        items = [
            ('X', "X", "Scaling to x axis"),
            ('Y', "Y", "Scaling to y axis"),
            ('Z', "Z", "Scaling to z axis")
        ]

        return items


class SCALING_OT_selected_objects_by_vert_location(bpy.types.Operator):
    bl_idname = "scaling.selected_object"
    bl_label = "Scaling selected object"
    bl_options = {'REGISTER', 'UNDO'}
    
    location: FloatProperty(
        name = "location",
        description = "",
        default = 1.0
    )

    axis: EnumProperty(
        name = "axis",
        description = "",
        items = axis_list_fn
    )

    def execute(self, context):
        bpy.ops.object.mode_set(mode='OBJECT')

        # Search selected vert from selected objects
        # Get vert location and vert belongs obj
        w_slct_verts_subed = None

        # 選択されてる点の座標をworld座標系で取得
        for obj in context.selected_objects:

            # 他の種類のオブジェクト(特にアーマチュア)を選択していても一緒に拡大できるように
            if type(obj.data) != bpy.types.Mesh:
                continue

            w_mat = obj.matrix_world

            for vert in obj.data.vertices:
                if vert.select == True:

                    # 1点よりも多く選択されていたらエラー
                    if w_slct_verts_subed != None:
                        self.report({'WARNING'}, "Operate is canceled. More than one vertex selected.")
                        return{'CANCELLED'}

                    w_slct_verts_subed = w_mat @ vert.co

        
        # 選択が無ければエラー
        if w_slct_verts_subed == None:
            self.report({'WARNING'}, "Operate is canceled. Vertex not selected.")
            return{'CANCELLED'}

        # axisの値に応じてscaleに使う値を変更
        scale = 1
        if self.axis == 'X':
            # 0除算対策
            if w_slct_verts_subed[0] != 0:
                scale = self.location / w_slct_verts_subed[0]
            
        elif self.axis == 'Y':
            if w_slct_verts_subed[1] != 0:
                scale = self.location / w_slct_verts_subed[1]
            
        elif self.axis == 'Z':
            if w_slct_verts_subed[2] != 0:
                scale = self.location / w_slct_verts_subed[2]
        

        # 各オブジェクトの原点が(0, 0, 0)にあるとは限らないので，3Dカーソルを(0, 0, 0)に移してそこを中心に拡大
        # 最後に戻すために現在の設定を取得
        temp_cursor_loc = Vector(context.scene.cursor.location)
        temp_pivot = context.scene.tool_settings.transform_pivot_point

        context.scene.cursor.location = (0, 0, 0)
        context.scene.tool_settings.transform_pivot_point = 'CURSOR'
        bpy.ops.transform.resize(value = (scale, scale, scale))

        context.scene.cursor.location = temp_cursor_loc
        context.scene.tool_settings.transform_pivot_point = temp_pivot

        bpy.ops.object.mode_set(mode='EDIT')

        return {'FINISHED'}


class SCALING_OT_scaling_by_two_selected_verts_location(bpy.types.Operator):
    bl_idname = "scaling.by_two_selected_verts_location"
    bl_label = "Scaling selected object by two selected verts location"
    bl_options = {'REGISTER', 'UNDO'}

    location: FloatProperty(
        name = "location",
        description = "",
        default = 1.0
    )

    axis: EnumProperty(
        name = "axis",
        description = "",
        items = axis_list_fn
    )

    def execute(self, context):
        bpy.ops.object.mode_set(mode='OBJECT')

        # Search selected vert from selected objects
        # Get vert location and vert belongs obj
        w_slct_vert_co_1 = None
        w_slct_vert_co_2 = None


        # 選択されてる点の座標をworld座標系で取得
        for obj in context.selected_objects:
            
            # 他の種類のオブジェクト(特にアーマチュア)を選択していても一緒に拡大できるように
            if type(obj.data) != bpy.types.Mesh:
                continue

            w_mat = obj.matrix_world

            for vert in obj.data.vertices:
                if vert.select == True:
                    if w_slct_vert_co_1 == None:
                        w_slct_vert_co_1 = w_mat @ vert.co

                    elif w_slct_vert_co_2 == None:
                        w_slct_vert_co_2 = w_mat @ vert.co
                    
                    # 2点よりも多く選択されていたらエラー
                    else:
                        self.report({'WARNING'}, "Operate is canceled. More than two vertex selected.")
                        return{'CANCELLED'}

        # 選択が無ければエラー
        if w_slct_vert_co_1 == None:
            self.report({'WARNING'}, "Operate is canceled. Vertex not selected.")
            return{'CANCELLED'}
        # 選択が1つならエラー
        if w_slct_vert_co_2 == None:
            self.report({'WARNING'}, "Operate is canceled. Not enough vertices selected.")
            return{'CANCELLED'}

        # axisの値に応じてscaleに使う値を変更
        scale = 1
        w_slct_verts_subed = w_slct_vert_co_1 - w_slct_vert_co_2
        
        if self.axis == 'X':
            # 0除算対策
            if w_slct_verts_subed[0] != 0:
                # fabsで絶対値にすることでx軸方向の長さに変換
                scale = self.location / math.fabs(w_slct_verts_subed.x)
            
        elif self.axis == 'Y':
            if w_slct_verts_subed[1] != 0:
                scale = self.location / math.fabs(w_slct_verts_subed.y)
            
        elif self.axis == 'Z':
            if w_slct_verts_subed[2] != 0:
                scale = self.location / math.fabs(w_slct_verts_subed.z)


        # 各オブジェクトの中心へ拡縮するため，ピボットを変更しつつ拡縮
        # 最後に戻すために現在の設定を取得
        temp_cursor_loc = Vector(context.scene.cursor.location)
        temp_pivot = context.scene.tool_settings.transform_pivot_point

        context.scene.tool_settings.transform_pivot_point = 'MEDIAN_POINT'
        bpy.ops.transform.resize(value = (scale, scale, scale))

        context.scene.cursor.location = temp_cursor_loc
        context.scene.tool_settings.transform_pivot_point = temp_pivot

        bpy.ops.object.mode_set(mode='EDIT')

        return {'FINISHED'}


# メニューを構築する関数
def menu_fn(self, context):
    self.layout.separator()
    self.layout.operator(SCALING_OT_selected_objects_by_vert_location.bl_idname)
    self.layout.operator(SCALING_OT_scaling_by_two_selected_verts_location.bl_idname)

classes = [
    SCALING_OT_selected_objects_by_vert_location,
    SCALING_OT_scaling_by_two_selected_verts_location
]

def register():
    for c in classes:
        bpy.utils.register_class(c)
    bpy.types.VIEW3D_MT_edit_mesh.append(menu_fn)


def unregister():
    for c in classes:
        bpy.utils.unregister_class(c)
    bpy.types.VIEW3D_MT_edit_mesh.append(menu_fn)


if __name__ == "__main__":
    register()