Just thought I'd followup... I ordered a 20" from Thor's site
The bike showed up today so I slapped it together and am quite happy with it (now I just need the rest of this snow to melt).
The height with the seat as low as it goes is exactly what I was looking for. So in the end an 18" would have probably been the "proper" fit, but I'm completely happy with the 20" as well.
Thanks for the help!