I would have thought that this would be easy to do, but I can't figure it out...

I have many small pics that I need to scan. They vary in size from 1"x1" to 3"x4". Is there a way that I can get my scanner to scan only this small portion (and not scan the full "letter size" length)... this would save me a huge amount of time. (ie. It takes the scanner about 30 seconds to scan full page -- 1" should take less then 3 seconds). Ideally I would like the scanner to automatically determine the size of the picture, but this is probably a scanner or hardware specific feature -- which mine may not have.