Sizing a bike based on what it 'looks' like is silly and pointless. Yes, going to the LBS to get an idea of the size you will need will work- although I think that it's a slap in the face to the LBS to try stuff out with no intention of doing business with them.
I would suggest that buying the Diamondback from a good LBS will be better in the long run- they will set you up on a properly sized bike and will take care of ya.
By the way, IIRC Bikes Direct has a decent set of instructions to determine what size will work for you.
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"I tell you, We are here on earth to fart around, and don't let anybody tell you any different." - Kurt Vonnegut jr.