In Starsector/starsector-core/data/config/settings.json there are some values you can change.
"minBattleSize":200, # in supplies-to-recover. min value for settings slider
"defaultBattleSize":400, # in supplies-to-recover. starting value for settings slider
"maxBattleSize":400, # in supplies-to-recover. max value for settings slider
"stationDeploymentBonusMult":1, # battlesize is increased by (station cost * this) deployment points
"minFractionOfBattleSizeForSmallerSide":0.4,
Specifically, maxBattleSize can be increased to larger values, although putting more ships on the field may (or may not) have an adverse effect on combat performance depending on your computer. You could set it to 1000, and then send the in game slider setting all the way up to 1000, and set minFractionOfBattleSizeForSmallerSide to 0.5 while you are at it, and you'd be guaranteed a 500 DP deployment each time, for example.