I looked but couldn't find what skills are modified because of size differences and by how much. I think it was all dex and strength based skills, but I'm not sure the amount.
The only skills that are modified directly by size are Fly and Stealth. Fly is modified by +/-2 for each size category below/above Medium and Stealth is modified by twice that number.
Intimidate gains +4 if the user is larger than the target or -4 if the user is smaller.