I am a little over 5ft 10 inches and went with the medium, mainly because I feel the large caters more towards a 6 footer. Did I make the wrong choice? I am not an experienced rider and this is my first adult bike (I'm 36) so my most recent comparison was when I was 16.

I love it and it feels good, and I'm a slender frame (about 165lbs) so do I need to go to a large? Will REI let me swap it out? I've only put about 20 miles on the bike so far...