I bought my girlfriend a Trek 4300 as a surprise today. I had to guess on what size frame to get her and just wanted to check and see if I got the right size, I am hoping someone here has some experience with the WSD bikes. My girlfriend is 5'6"ish, maybe 5'7". I got her the 16" WSD Trek 4300. Do you think she will need the 18"? Luckily the bike store will exchange it if it doesn't fit her. Please give me some feedback.