400 DP is available from options right away (and is preserved on version update for people who have set it earlier). More can be easily set in config.
If changing battle-size for your advantage is considered too exploit-y, it could be fixed once on campaign start.
Changing battle-size during play is massively exploitable, lol.
Here, just for fun, try making the range run from 50 to 1000. At 50, you can chain-kill practically any fleet in the game, at 1000, your CPU will melt, but you can see what happens when the AI isn't gimped on deployment. 1000DP that's reasonably playable has always been the goal of my AI project, lol.