My daughter is three and one half, and has been urinating on the potty for some time. However, she seems to be afraid to have a bowel movement without a diaper. Nothing we've tried seems to be working. We have to figure something out fast, because she's almost outgrown the biggest available size diaper!
While it may seem that many toddlers you know are already completely trained, lack of bowel control isn't considered abnormal until after four. Just as there is a wide variability in when children walk and talk, so is the case with toilet training.
There are two main reasons why a child hasn't achieved this control. The first is that she can't. Developmentally, she hasn't mastered the ability to coordinate certain signals with the required muscular control. No one can hurry that process along.
The second reason is that she won't, and there could be many reasons why. She may have genuine fears about even sitting on the toilet, especially the adult variety, which then makes a loud noise. She may be trying to assert her independence around this particular issue. She may not be temperamentally ready; it usually needs to bother toddlers that they are walking around in soiled diapers before they are motivated to do anything about it.
Regardless of the reason, the advice to parents is the same. Don't punish. Don't insist. Don't assert your authority. Instead, let her know how much it would please you to see her trained. Gently require her to be more involved in the changing process, having to fetch a new diaper, for example. Reward her with a sticker chart or even a hug for her successes, and let her use a child size toilet where her feet can easily reach the floor. After that, it's up to her.
Oh, and don't worry about diaper or pull-up sizes. They exist to fit adults and everything in between. You just may need a different brand.