It might be Picasa that is messing you up!
Be sure to have Shrink to Fit checked not Crop to Fit on the print screen.
You also have to print each size on its own, so the first the 4x6 and then the 3x5 on a different print command, ect. Ad be sure you are selecting the size you want on the print screen as well as cropping them the right size!
Try those things, hopefully they help!