What vesion of excel are you using? I assume when you speak of size you mean Mbytes and not number of sheets and/or rows and columns?
I work with excel 2002 and win2000 operating system. I sart to experience difficulty when I hit the 15Mbyte mark. If I clean up the sheet ie reduce the number of links, macros, sheets and complex formuals I can go even bigger.
I'll post some usefull excel links here later when I get a chance.