I use image shack, works great, no real limit and easy to use.
As to size, you can post shots any size. I have posted some as big as 1920x1200, but it takes up too much room on the forum. Generally, 1024x768 is best. i.e:
When you upload to imageshack, it gives you the option to resize the shot. It will even give you the direct code, so you can just cut and paste directly into your post, as I did here.
I would suggest you also convert your shots before you upload them. If you take shots in-game, they are saved as BMPs which can be as big 5-6 MB per shot. You can use a program like "Paint" to re-save the shot as a JPEG which will reduce the size to as little as 30-40 kb, making easier to upload to ImageShack and less of a hassle for users here who have to download the shot.