Yeah yeah I know everyone's sick of these. I'm just after some advice on a general size to start looking at. But I'm looking to buy used and bike shops are a bit disinterested in sizing me up properly.

So I'm 168 cm tall or about 5'6" and a bit. What's a good frame size or frame size range to start looking at?

It's not strictly for mountain biking but more mostly commuting with some trails. So I'm looking at a hard tail or fully rigid. So I think that also means I might go a tad large than if I was using it purely for mountain biking. True?

Thanks for any help.