Alberto,
If you check the file size after the file is uploaded, you are sort of defeating the purpose since you normally want to warn a user of a large file to avoid unexpected delays while the very large file is uploaded.
The client side would be the best place for this, however javascript cannot access the filesystem and the only other way i know of doing it is to embed an ActiveX object into javascript and now-a-days browsers will warn you and some won't support ActiveX at all.
Perhaps some else knows a way around this? Otherwise you are stuck with server side only.
Regards
Bill