How to get size property like Vector 3?

Try the Type.GetFields() and Type.GetProperties() methods. You'll need to use System and System.Reflections namespaces.

 Type myType = typeof(Vector3);
 FieldInfo[] fields = myType.GetFields();
 PropertyInfo[] properties = myType.GetProperties();
 int amountOfFieldsVector3 = fields.Length;
 int amountOfPropertiesVector3 = properties.Length;
 

Not sure if that will work though, because the Vector3 is not a class but a structure.
Info about GetFields(): https://docs.microsoft.com/dotnet/api/system.type.getfields?view=net-5.0

About GetProperties(): https://docs.microsoft.com/dotnet/api/system.type.getproperties?view=net-5.0#System_Type_GetProperties_System_Reflection_BindingFlags_

 GameObject.Transform.LocalScale 

Should work.

There's no real correct way to do this, however one way I can think would be to count the actual bytes in the struct and compare them with the size of a single element;

 public static int NumberOfElements (System.Type a_ObjectType, int a_SizeOfOneElement)
 {
     //Use marshalling to get the size of an allocation for this type (Vector3 would be 3 * sizeof (float)
     int objectSize = System.Runtime.InteropServices.Marshal.SizeOf (a_ObjectType);
     //Divide by the size of a single element to get the number of elements
     return objectSize / a_SizeOfOneElement;
 }
 
 ...
 
 //The Vector classes are made up of floats, so pass that as the size of an element
 int elementCount = NumberOfElements (System.Type.GetType (property.type), sizeof (float));

Haven't tested this, but I can't think of any other way really ( @Ermiq has the most 'correct' method, but the Vector classes' fields don't necessarily map the way you might expect).