I have a canon printer too, although I think your problem is not with the printer. I print different sized of photos on my printer all the time.
How exactly do you go from Picasa to the printer? Do you download them? Do you put photos in a program like Word or PSE before you print? What's the resolution of the files? In the printing dialogue, did you uncheck "fit to page" or something like that?
I always drag photos into a file that corresponds to the photo size. For example, if I want to print 3 small photos on my 8x10 photo paper, I make sure my document size is 8x10, arranging photos so they all fit, and print to a 8x10. If your document size (not photo size) is different than your paper size, then you'll have a lot of problems. I need more information so I can help you.