You have to be careful with bobs on a round face. Your head can appear rounder with a cut that's all one length or that falls at the jawline or above it. Your best bet is the uber-popular "long bob." Long bobs are usually at least 2 inches longer than the jawline and flatter a round face because of the length.
If you have fine, straight hair and a round face, this long bob as seen on Tori Spelling is a great hairstyle option that looks wonderful flat ironed or curled (Tori's hair here is curled by winding hair around the barrel of a curling iron).
For a more modern bob, make sure the back of the cut is shorter than the front and the ends have texture and are wispy. Texture and an angle add interest to what can be an otherwise boring bob.
See more long bob hairstyles
More on face shapes: