You can't calculate that, any more than you can calculate the proper price of bread. It's all up to how people trust and use bitcoin. A 2 mb cap would certainly increase use, and lower fees, but how much is something that can only be seen as it happens. Could be more room will lead to lower fees and thus higher rates of use, but it could also be that enough people are leery of the fee/slow transaction issue that they might avoid it.
What's really important is that raising the size will get people used to the idea of raising the size in the future. Kind of like how people don't even pay attention any more to the fact that the government hasn't had an actual budget for the last however many years, because that's just how it is now.