Check camera settings first...
Check on your camera's menu - there should be a setting you can change to adjust the pixel size of the pics you take. The higher the pixel number, the larger the pic. There may also be a resolution setting that will determine the quality of each pic, the higher the res the better the quality of the pic but you will get less pics per usage because it takes more memory for higher res. IF you're taking pics you may want to enlarge, higher res is better but not always.
As far as changing the pic size AFTER you've downloaded them from the camera, use a photo editing program like Adobe's Photoshop. Most likely, you got some photo editing software with the camera unless it is a really low priced one (didn't want to say 'cheap'

).
Anyhow, in the software for the photo editing, you simply choose which picture from the downloaded pic folder you want to work on, select "Image Size" (or whatever the software describes) and adjust from there.
It's really not hard at all - you just need to use the appropriate software and I'm wagering you have already got it with the camera. It should be on a disc included in the camera outfit.