hun.
Firstly i am 5'11" and was quite worried with DD1 that i had such a small bump compared to other women but my OB explained i had {---------------------} this much room when others only had {------} this much so please don't stress TOO much about this until you have the scan and know exactly whats going on.
Both my girls were only 6lb11oz so not tiny tiny but smaller than the average and they are perfectly healthy.

In regards to breech have you checked out the info on spinning babies?
Lots of bubs can turn before they are due, so try not to worry yourself too much, arm yourself with as much information on breech birth and spinning techniques as you can and deal with what your given based on your knowledge and comfort.
Will your OB support a vaginal breech birth? It can be done but only if your comfortable with it. (see my DD1's birth story).

All i can suggest is arm yourself with information, discuss things with your OB and go with what your comfortable with, if bub does not turn it's not the end of the world, you make a decision based on what you feel is right for you and your baby at the time.. good luck with it all, hoping bub turns and is a nice healthy size for you.

I'm not saying because vaginal CAN be done that it should but if it's something you want to consider it's good to know it CAN be done, but you need to be aware of the risks for both C/S and Vaginal.
Had i of known DD1 was footling breech there is every chance i would have had a C/S based on the risks, it's worth knowing details on both as some OB's like to use scare tactics but ultimately it's up to you and what your comfortable with.