Substrate Definition (size) for fabrication in ADS momentum

We have designed patch antenna in ADS momentum. When we went to manufacturer, to manufactured it, he asked us to give him 'gerber file' . We created the gerber file of our design but now manufacturer is asking to give me file with definite size of substrate. As in momentum the size of substrate is infinite so how can we define size of substrate in ADS momentum.

In Momentum, the size of the substrate is infinite. Always.

The size of the bottom ground is also infinite if you use the //// GND //// ground boundary condition. If you want to simulate with a finite ground, you can draw that and map it in the EM layer stackup and then use FreeSpace boundary above and below. But for patch antennas that should not make much difference, as long as the ground is "big enough".