I'm a casual rider and of average height with I guess what you would call normal proportions. As a kid a great many of my bikes were purchased solely on if I could straddle the top tube and reach the pedals. I'm almost 57 now and I have enough of an understanding of what works for my needs I don't know that I would have a lot of reservations buying online especially if there was a decent return policy.
Like everything bicycles though, this isn't a one size fits all thing literally and figuratively. Like others have stated, research what you need well. Especially if you are planning on going from something like a mountain bike to road bike in which case I'd strongly suggest a test ride on something at least similar to what you are looking at.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke