If you open the file in Photoshop (or equivalent), you can Click on Image > Image Size. Mine initially tells me the size of the current file in pixels, but I can select inches in the drop down list. From there (same screen) I can change the size of the image. Just make sure "Constrain proportions" is checked. You can also change the dpi on this screen. Just note that if you change the current picture from 72 dpi to say..... 300 dpi... it will have to extrapolate the missing pixels, and the end product may appear a bit fuzzy (good to experiment with in your spare time).
Pixels: Most monitors can display a max of 900 x 600 pixels.
DPI: The higher the dpi, the more detail is preserved as you increase the size of the image. If you want to stretch out this 8.5 x 10" image to 20" wide, you want a high dpi. I'm still trying to figure the formula myself for a good dpi vs size I can increase the image to. But, most pictures I deal with from other photographers are around 8.5 x 10" and 300 dpi, on average. Not saying it's right or wrong, just passing personal info.
JPG vs TIF: JPG is a good small file size for images, but I usually only save a COPY of the edited version as a JPG. JPG's are a lossy file type, which means if you intend on editing it a lot (as I do in my own images), you lose a little more information each time you save it. TIFs do not have that problem, but the file size is much larger. So, for myself, I edit the original and save as a TIF, then save a copy (or save for Web if that's what it's for) as a JPG. That way, if I need to edit something again, I have my original, non-lossy file.
PNG: Good to save as a PNG if you want to make the background (such as a ClipArt) transparent. Just remember to delete the original "background" layer.
Ok, probably more information than you need at the moment, but something you can file away for later. And yes, I break my own rules quite often (editing the heck out of a jpg only because I didn't think I would edit that particular file again).
A third of my work is to be viewed on the web, so using JPG is usually just fine, as the loss of data isn't always that noticeable. A third is printed color photos... TIFs definately, as these are sometimes blown up. The other third is printed black/white photos... again TIFs so I don't end up with artifacts and other things that can detract from the image I'm trying to convey.
