bramcor, it will work until around 2.100.000.000. A value of 3.000.000.000 will fail loading. (but does not crash!)
Also negative values work perfectly fine and don't crash the game, even not in battlescape. They can go as low as -2.100.000.000
Technically they can go from -2,147,483,648 to 2,147,483,647
(or -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807 if you compile it in 64-bit).
And things like amount of bases/soldiers/crafts/etc are only limited by your available memory.