I am currently a student in a Mechanical Engineering school.  I graduate in a year and a half and plan to enlist in the Marines OCS program.  As an officer will I be able to be on the ground as a combat unit?  Or being a MechE graduate they are going to stick me wherever they like.  I am also 6'4" and some guy told me I'm too tall to be a combat unit.  Is that true?    

All newly commisioned officers have to serve their first tour in an infantry unit.
Don't worry about your degree - the Green Machine doesn't have much use for mechanical engineers.
Now the Navy on the other hand...

Quoted: All newly commissioned officers have to serve their first tour in an infantry unit. Don't worry about your degree - the Green Machine doesn't have much use for mechanical engineers. Now the Navy on the other hand...

No they don't, what all newly commissioned officer do have to do is go to the 6 month Basic School which emphasises duties as a rifle platoon commander.

Your degree has nothing to do with what you do in the Corps, your place at around the 2/3 mark in your Basic School class determines what your MOS is. We use what is know as a quality distribution spread it which the company is divided into thirds and the top of top third pick, than the top of the middle, than the top of the last.  From there it goes to the 2nd in each thirds all the way till all the MOSs are assigned.  

So if you are at the top you get to pick you're own MOS?  

Pretty much, unless you are blind as a bat and want to fly.  

STLRN,
Check your email.  Thanks.  
-Justin