I should have included I'm using FMP 6 on an intel 10.6.x machine.
You can use the Adjust Window script step, resize to fit, to set the appropriate Window size. I must admit that I have no idea whether or not FMP 6 supports this step. You would do well to upgrade all the way to FMP 11.
FMP6 doesn't seem to be able to use that command. I'd also need to buy four copies for the office which ain't happening this year or even next, from the look of things. Besides, I'm waiting for any slight improvement in the layout tools. :>)