Shopping for toothbrushes is like shopping for shoes in a way; the store always seems to be out of the size you want. That leaves you looking at different brush sizes, some possibly larger than you want to use. Does that really matter, though? When you compare small and full brush heads, for example, the difference seems very small. There are times when brush size matters, but in the end, it really is a matter of choice.
One area where the size really does matter is in the age of the person using the brush. Young children with small mouths definitely need a smaller toothbrush, and for really young kids, look for brushes made especially for children. Once a person reaches the teen years, then brush head size becomes more of a personal preference, though depending on whether the teen is growing quickly or not, a smaller brush head may still be more appropriate for a while.
The smaller the brush head, the easier it is to reach behind the back teeth, especially if your mouth doesn't have a lot of space. Not everyone has a lot of room for a brush to maneuver around the back teeth, which means that if you don't use a brush that will fit back there, you might not be able to brush effectively. You might miss spots, for example.
Note, though that brush head size is only one variable here. Some brushes have extended bristles at the tip, and these can help you reach those far-back areas.
Another feature to look at is the angle of the brush. Just as extended bristles can help reach spots, an angled brush head may make it easier to reach behind the back teeth even if the brush head is a little too big to fit comfortably back there.
If all you can find are large brush heads, look for ones that are angled, that are narrower than usual (even if they're long), or that are made for children (no one needs to know the brush is for you). If you can get a brush that allows you to reach all of the spots in your mouth that need brushing, you'll do fine.
If you're still having trouble finding a brush that you feel comfortable with, call your dentist to ask about recommended brands and features. Your dentist will also likely have sample brushes; take one home and see if that works.