There are oceanic fosssils high up in the Andes mountains and other mountains, so it is hard to say what was originally under water and what was dry. But yes, I too believe that there was one huge landmass that was catastrophically altered.
Some creationists believe that much of the current strata was deposited at the flood. The deep water, under high pressure and temperature, had huge amounts of dissolved minerals, and when it erupted many rock layers were formed. This would explain areas where you have many layers of coal interspersed with rock. They would say a mat of vegetation was covered by an eruption, more plant life washed over the area and another eruption, etc. All this during perhaps months, not millions of years. But this would mean that many areas that we now have with all sorts of rock layers were not part of the original pre flood land mass.
Some people believe the land separation happened at Babel, and Gen 10-11 when the earth was divided was not just language but physical ( days of Peleg) as well. I don't think we can do more than speculate. I personally think there are enough accounts of a great Atlantean civilization ( Atlantis) which was flooded to say there was more than one catastrophic event, although none on the level of Noah's flood.
You can't say how much land there was, I mean, even if water was added at the flood perhaps when the mountains pushed up the ocean basins sank lower and could hold more water. Interesting question though.