Default size is 4, which can be expanded to 6 with upgrades.
This tweak increases default size to 6, which will increase to 8 with the base upgrades.
(Based on xcom 1 long war numbers)

The UI only supports up to 6 on the screen.According to pcgamer, it would support 8 but that appears to be incorrect.

If you want more than the 6 + the 2 from upgrades, I can provide optional files.
When it's set to higher than 8 in game, you'll have 9 soldiers with you at your spawn point,
the rest of your soldiers will all appear on one square a little bit from your starting location.

Screenshots provided to show examples.

Instructions

unzip and place the defaultgamedata.ini file in steam > steamapps > common > xcom2 > xcom game > config
Overwrite the file when the prompt comes up.


Game updates may revert these changes, so if it happens, you'll need to reinstall.