15Mb and beyond is fairly normal size wise.
Well written spreadsheets can still work well at 2-300Mb in size on a
standard corporate workstation.
The big advice would be to work locally on your C: drive if possible and
copy to the network as a separate activity. Opening big files over the
network can be painful in Excel.
Your crashes are more likely caused by add-ins or file corruption than
pure size/complexity.
Cheers
Simon
Excel development website:
www.codematic.net