The LHT can take up to 45mm tires so you'll have no troubles on dirt/gravel roads, but the BB is quite low so that can be a problem if you go offroad.
As for the 56 vs. 58 question I'd compare the effective TT on a bike you are comfortable on and then ensure the stand over is not too high. If you don't have such a bike talk to a LBS to get sized.
Here is some Surly sizing advice.
As for the search I'm not suggesting you don't ask new questions just balance the effort req'd for conducting a search against the effort of board members answering the same questions again and again. There is a happy medium in the middle somewhere.
I looked at the Surly sizing advice link. It is pointing me toward the 56cm. My Trek 1500 is a 58 and fits wonderfully for aggressive road riding. I think the biggest problem will be finding a seat that loves me. My bony butt can't handle a brooks or any of the other no to low padding seats. The Surly is at the top of my heap right now. Thanks for the link.