I have a Pace500 on order for myself (can't wait for it to ship!). Large frame is was the obvious choice for me due to my size (6'4" and 260 lbs). Looking at the Pace500 Step Thru as a possibility for my wife, who is 5'6" - and I'd say "average to long legged" for her height.. and 150 lbs or so.. Per the Aventon website, the small frame size says 4'11" to 5'8".. and the medium frame size states 5'8" and up.. I lean towards the medium frame size for her.. as I'm thinking that frame size will keep her from having the seat post super high as it might be on the small frame size. (My logic says that for the small frame size to accommodate a 4'11" person, the seat tube can't be very tall or it would be too tall for that height?) I tried to make "comparable" measurements on her existing Schwinn non electric step thru hybrid bike.. ===> some match closer to the small frame Pace500 Step Thru.. and others match closer to the medium frame.. so not sure it helped that much. If this were a "non" step thru model like mine.. I might err on the smaller frame size due to stepover height.. but.. given this IS a step thru model, that isn't an issue. Any guidance would be appreciated!