6GB on disk could expand to as much as 30-60GB or even more in memory.
There is no single simple way of calculating this as it depends on a number of factors. You data model and the various cardinalities of each field play an important role.
The Windows Task Manager gives you a good indication of the memory footprint of an application when loaded. See how much memory is free before you load your 2GB app and after you have loaded it. This will give you some idea of the expansion factor.