S.T.
I am going to put my two cents in since you put the question out there. Some people may want a big family, but some jsut can't do it. If you feel emotionally that you can't do it, than you probably shouldn't. It isn't fair to the future children and it isn't fair to the exsisting.
My mom came from a family of 18 she knows all of the kids but mainly because she would take us over ther because most of the kids are my age or younger, so it was like they were my cousins, and I know that my aunt felt as though she was the one raising the younger ones. She had to grow up fast and that just isn't fair. I know about being part of a family you have to give things up sometimes, but I just don't think one of them should be your teenhood.
Anyway I don't know how big a family you were thinking but if you already feel that you have too much on your plate, your older kids will probably feel obligated to take hold and help. And they shouldn't have to do that they are kids.