Create a quad mesh asset in Unity. Place this script in the Editor folder.

CreateQuadMesh.cs

using System;
using UnityEngine;
using UnityEditor;

public class CreateQuadMesh : Editor {
  
	[MenuItem("Assets/Create/Quad Mesh", false, 10000)]
	public static void Create ()
	{
		Mesh mesh = BuildQuad (1, 1);
		string name = "Quad Mesh";
		mesh.name = name;
		AssetDatabase.CreateAsset (mesh, String.Format ("Assets/{0}.asset", name));
		EditorUtility.FocusProjectWindow ();
		Selection.activeObject = mesh;
	}
	
	private static Mesh BuildQuad (float width, float height)
	{
		Mesh mesh = new Mesh ();
		
		// Setup vertices
		Vector3[] newVertices = new Vector3[4];
		float halfHeight = height * 0.5f;
		float halfWidth = width * 0.5f;
		newVertices [0] = new Vector3 (-halfWidth, -halfHeight, 0);
		newVertices [1] = new Vector3 (-halfWidth, halfHeight, 0);
		newVertices [2] = new Vector3 (halfWidth, -halfHeight, 0);
		newVertices [3] = new Vector3 (halfWidth, halfHeight, 0);
		
		// Setup UVs
		Vector2[] newUVs = new Vector2[newVertices.Length];
		newUVs [0] = new Vector2 (0, 0);
		newUVs [1] = new Vector2 (0, 1);
		newUVs [2] = new Vector2 (1, 0);
		newUVs [3] = new Vector2 (1, 1);
		
		// Setup triangles
		int[] newTriangles = new int[] { 0, 1, 2, 3, 2, 1 };
		
		// Setup normals
		Vector3[] newNormals = new Vector3[newVertices.Length];
		for (int i = 0; i < newNormals.Length; i++) {
			newNormals [i] = Vector3.forward;
		}
		
		// Create quad
		mesh.vertices = newVertices;
		mesh.uv = newUVs;
		mesh.triangles = newTriangles;
		mesh.normals = newNormals;
		
		return mesh;
	}
}