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I ride a 19 inch Rocky Mountain Blizzard (23.4 inch top tube mountain bike) and am very happy with it.
I'm in the market for a new flat bar 700c touring bike and like the riding position of my blizzard (except perhaps I'd have my bars a little higher than on the blizzard...but that's not the issue here).
Normally I would buy a 56cm ROAD bike, but this is a very short top tube on a flat bar bike. So should I jump up to a 58cm touring bike frame (the frames are designed originally to run drops) so that I can ride with flats or stick to the 56cm which is 'my size' according to the shop and size charts.
a flat bar would suit a longer TT, as you suggest.
I'm 5'11" and run a 57cm TT on my flat bar, with a 12cm stem. I'd say a 57-59cm TT would be about right fot you, then adjust the stretch/rise with the stem.
Have you looked at Thorn cycles who do some frames in a longer top tube for this reason
Or look for a used hybrid frame on ebay
Or if you after A bike then the big manafacturers do makw what you want. I was oggling a carbon flat bar road bike this afternoon
But I think your right to get the geometry you know you want