I am naturally large from using the race builder, this gives me
Large (7 RP): Prerequisite: Humanoids taking this quality must have the giant subtype; Benefit: Large creatures gain a +2 size bonus to Strength and a –2 size penalty to Dexterity. Large races take a –1 size penalty to their AC, a –1 size penalty on attack rolls, a +1 bonus on combat maneuver checks and to their CMD, and a –4 size penalty on Stealth checks. A Large creature takes up a space that is 10 feet by 10 feet and has a reach of 5 feet.
If I were to be under the effect of Reduce Person, what exactly would my size modifiers be?
This spell causes instant diminution of a humanoid creature, halving its height, length, and width and dividing its weight by 8. This decrease changes the creature's size category to the next smaller one. The target gains a +2 size bonus to Dexterity, a -2 size penalty to Strength (to a minimum of 1), and a +1 bonus on attack rolls and AC due to its reduced size.
My first thought would be that they are added together and thus cancel each other out because medium creatures have no size bonus, but I've heard that's wrong. What's the net effect?