Rest

Hi,

I recently read an article that said the amount of rest a body part needs is proportional to its size. For example: your forearms are small, so they can recover by about 1 rest day. Your legs are bigger, so they may need say 3 days rest. I usually do a split program like this: Monday, Wednesday, Friday, alternating between working my upper body and legs each time. I am wondering if I am allowing enough rest for the bigger muscle groups like legs and back? I have a suspicion I am overtraining because I am sometimes still a bit sore when I go to train the second time. How much do you guys rest for your different body parts?