cant play it on full screen

Hi,
I took an AVI file and edited it on Premiere.
After that I made another avi file from it and tried to play it on full screen. Didnt work.
All I see is a small size video with black empty spaces around it.
I tried everything (raised resolution on premiere project) but nothing seems to work...
The original video has 320x240 and plays ok on full screen mode. But my edited video (at same resolution or higher) doesnt work in the same way...
Anyone could please help me??
Thanks!
Ricardo