Where to begin? First, check around locally and see who is hiring, right now just finding a place to apply to may be a tall order. Having a degree will help some, but the economy is tough on everyone right now and few places are looking to hire en masse right now.
31 is not too old here, but it varies by department and prior .mil or le experience.
Physical size is not the only determining factor when working the streets. We have a Kuk Sul Won master and 5th degree blackbelt who is about 5'3 140, not that anyone would ever guess by looking at him in uniform. But, he knows how to establish command presence, and has the skills to back it up if necessary. It is all about being confident and taking control or a SHTF situation quickly and decisively, without pissing everyone off even more, or taking it too far. It is a delicate balance to manage, but Officers of all sizes are successful when they know how to manipulate shitty situations.
I have been on with my dept since 07. City pop is about 135k-150k, and we have 187 Sworn Officers. I have a B.S in Criminal Justice/Police Science and double majored in Political Science. Worked in Juvenile corrections the last 18mo of college and quit as soon as I knew I was in the academy. I worked 2 years on patrol and have been working as a plain-clothes street crimes officer for about a year now, and it is the best job in the world. Some nights/days you will be like WTF???? am i doing this for, and others you will be like WTF!!! that was freaking awesome. Sometimes you aren't paid enough, and sometimes you will love it so much you would almost do it for free. You get to see a lot of things no one else will, and a lot of things you wish you never had. All in all, you cant beat it if it is something you have a passion for, just be prepared for it to be totally different than you expect it to be, and at the same time be prepared to have your eyes opened to what really goes on in the world.
Good luck to you, and let us know what you decide.