I don't put a lot of faith in the "ideal" frame size. As a full grown adult, I've ridden road bikes from 58cm to 66cm and was able to make them comfortable for me to ride on for long distance or what ever. My inseam is just over 34.5 and i'm only 5' 11" with long arms.

Do you have a previous bike that you are comfortable on? If so, what? and what size?

Even if you are going to buy online, at least go down to a shop and try one on for real to see what it feels like. The shop will adjust the saddle height for you and maybe let you ride it some.