Create fields of 1000, 2500 and 5000 map scale bounding width and height for every feature on active layer. Currently using UTM Zone 47N for tranformation.

pyQGIS_create_field.py

# layer to hold our current selected active layer
layer = iface.activeLayer()

#create new fields: skala1000w,skala1000h, ... 
layer.dataProvider().addAttributes([QgsField("skala1000w",QVariant.Int),QgsField("skala1000h",QVariant.Int),QgsField("skala2500w",QVariant.Int),QgsField("skala2500h",QVariant.Int),QgsField("skala5000w",QVariant.Int),QgsField("skala5000h",QVariant.Int)])

# save all the changes
layer.updateFields()


# expression to be executed into corresponding field
expression_skala1000w = QgsExpression('ceil(bounds_width( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\')))')
expression_skala1000h = QgsExpression('ceil(bounds_height( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\')))')
expression_skala2500w = QgsExpression('ceil(bounds_width( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\'))/2.5)')
expression_skala2500h = QgsExpression('ceil(bounds_height( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\'))/2.5)')
expression_skala5000w = QgsExpression('ceil(bounds_width( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\'))/5)')
expression_skala5000h = QgsExpression('ceil(bounds_height( transform(   $geometry  ,\'EPSG:4326\',\'EPSG:32647\'))/5)')
context = QgsExpressionContext()

context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(layer))
with edit(layer):
    for f in layer.getFeatures():
        context.setFeature(f)
        f['skala1000w'] = expression_skala1000w.evaluate(context)
        f['skala1000h'] = expression_skala1000h.evaluate(context)
        f['skala2500w'] = expression_skala2500w.evaluate(context)
        f['skala2500h'] = expression_skala2500h.evaluate(context)
        f['skala5000w'] = expression_skala5000w.evaluate(context)
        f['skala5000h'] = expression_skala5000h.evaluate(context)
        layer.updateFeature(f)

        
        
#tampilkan notifikasi
iface.messageBar().pushMessage("Sukses", "Menjalankan Ekspresi" ,level= Qgis.Success)