I know I can use the Javascript API to restrict uploads by filetypes and by maximimum size. Can I restrict the upload of certain filetypes by their size, e.g. No Excel files greater than 1mb and No CSV files greater than 500kb?

This is not currently possible. You can specify multiple filetypes, but you can only set one maxSize per session. So that max file size will apply to all filetypes you specify.