I have some things in common with you so I'll try to answer your questions, but you should search the forums as many of your questions have been answered.
Here we go:
I want to be an aviator and had a few questions about the selection process.
Would I know if I have been selected for a flight position before I agree to a commission?
Yes, you'll be selected for a pipeline prior to OCS. If you make it through the physicals at OCS and then again in pensacola, you'll continue on with aviation training.
Are there more flight opportunities in the Navy or in the Marine Corps?
I'll let one of the more experienced guys answer this as I have no idea.
Are aviator commission typically given to OCS graduates, or is it mostly the academy and ROTC?
There is no such thing as an "Aviator Commission." You commission as a Naval Officer. Period. That is your primary job and whatever your designator/MOS is, is secondary. Aviation is competitive, especially as of now with economy, but so are many of the other designators too, even more so. Commissions are given/awarded to those who either successfully complete OCS or make it through the Academy/ROTC.
Also another note, just because you get accepted to "try out" for Aviation, doesn't guarantee anything. I had people in my OCS class who never even made it into the beginning of aviation training in Pensacola. Some DOR'd or some just couldn't hack it and failed out.
I was an Aerospace Engineer as well too. Do your best and apply for the program and you'll never know what happens.
I am also worried I may be medically disqualified for flight school. I have a -2.00 prescription in each eye and I have also been a powerlifter since high school so I am a pretty big guy, 6'0 & 235 lbs. Not sure if there are size constraints for flight school.
I have bad eyes (soon to be fixed) but I am also an NFO and (well Student NFO) and love my job. Any "job" in aviation can't be beat....pilot or nfo, go with what you want. My eyes are worse than yours. Also i'm 6'5 and 236, not a powerlifter, but working towards some bodybuilding goals....there are limits for certain platforms and that's all a part of the "indoc" period while you're in pensacola. You'll find out what you're qualified, anthropometrically, to fly and what you're not.
If you have other questions, feel free to PM me and ask.