Hello, I am 6 feet tall with a 34 inch in-seam, would a Honda Rebel 250 be comfortable enough for me?  

You will probably be a little cramped on that bike.  Go to your local dealer and have a seat.  That will tell the story.  I started with the Shadow 750, I'm 5'8" with a 32" inseam.  The Rebel feels tiny to me now that I have been riding for a year.  

Waterfront You will not be happy on a Honda rebel and mostly likely will want a bigger bike within 3 months  

I'm 6feet 2 and have a 34 inch inseam also. The bike you mention is a bit small but you may not be able to buy exactly what you want brand new. Bigger bikes are not really harder to ride, they are more forgiving on the road because of weight and handling. If I were new at it:that was 47 years ago, I'd look for a 650 shadow or 650 or 750 yamaha street cruiser. They will allow you a lot more room to stretch your legs forward and lots better ride on the road. Smaller bikes get a lot more force on them when they have blow by on the highway like 18 wheelers and do way better on rain.
I'm owned 650's, 900's 1100's 1700's 1900's and bigger is almost always better. Used bikes with less than 40 k miles are usually still good for another 60k if they are well maintained and if you keep up the mtce yourself, it's not unusual to see 150 k on big bikes.
take care and play the game when you start to ride.....
someone is always going to run over  you if you expect them to see you or give them the chance,. Drive like everyone is stupid and you'll do well