What do you mean by "size"?
If you mean how many rows and columns, CTRL+END will take you to the "last
used cell" on a worksheet.
However, I suspect you may mean "file size". An individual worksheet is not
a file, so doesn't have a file size. You could get an idea of relative sizes
by copying each sheet to a separate workbook and saving, but it would only
be a very rough idea.