Photo printer problem

I have an HP 7760 photo printer; it can either be connected to the computer or used as a "stand alone" printer by inserting the card from my camera in it and printing pix that way. One list group I am on, several of the ladies there have raved about the Picasa photo editing program, so I downloaded that. Since I can now edit pix with Picasa, I finally put the program for my printer in the computer. Now, I cannot seem to be able to print pix from Picasa. It tells me that the paper size is wrong. I tried the troubleshooting, and there was something about how to get into "properties", but it didn't solve my problem. Can anyone give me a step by step solution? I have to tell you, you need to be very specific, as I am pretty dumb about this stuff. If anyone can, I'd appreciate it. If not, I will bite the bullet and call my computer man to come out and show me. *sigh*..........it's tough being a visual learner. LOL