To prevent that from happening I read you make "states," where it's saved and you can go back to it. It never worked for me, so if you'd like to look how it works exactly, google "Save to Channel with Gimp." Any ways, just trace around them to the best of your ability without grabbing too much of the green background as you can. When you have them completely traced, and there is a blinking line around them all. You finished your selection. You get two choices at this point, you can either cut them out and put them into a smaller picture, or you can clear whatever is left around them. I'll explain the first, than the second. So if you want to cut, simply right click, cut. What I do to find out exactly how big it is. I go into paint, make the window really small to start. Paste, and it should tell you the dimensions. Open a new gimp window paste it in there with the dimensions that Paint gave you.