at a certain point your seat too far forward will start to cause probs. i'd focus more on your stem and bar combo. ideally you wanna get your position over the pedals set for maximum effiency then alter your reach(top tube/stem/bar) to get the appropriate reach. of course it all depends on your leg length, but if all i knew was that you were 5'10", i'd pull a 54cm for you to try. the 56 might be too big, but you should be able to get close with it.