I spent a couple of hours searching for a script that returns the size of all the tables in a database. Most of the ones that you find use sp_msforeachtable but that is just for one table and you then need to use temporary tables to hold the values.
This article on the Database Journal by Ananth Kumar (AKA MAK) had this handy script that simply works by copy-pasting it as is. Read the linked article for details on viewing the size by schema.