I don't do much designing, but when I do I always check royalty free sites first. Here is a good one:
stock.xchng - the leading free stock photography site
Regarding dimensions, you usually are able to get the original resolution as created by the camera so this can be xxxx*yyyy, then its up to you to resize or do whatever to it, so it doesn't really matter. Obviously the bigger it is the better as it will give you more options on what you can use it for.