I just want to change one small thing, how much health the player has.

There must be a setting that determines what this is and how much it goes up as the player levels.

Anyone know what it is?

In the Geck, go to 'Gameplay', then 'Settings'. Filter by 'health'. Looks like endurance has a lot to do with it. I haven't messed with these settings myself, but other's have, in overhaul mods.