I refer you to "What is Crossfit?" in the "Start Here!" tab on the main page. This document provides an excellent explanation to your questions.
In addition, you might consider inserting ME (Max Effort) workouts on the 2nd day of the 3 day CF cycle if you are seeking to increase your strength primarily.
Eg. Day1-CF MetCon. Day2-ME. Day3-CF MetCon. Day4-Rest, repeat cycle.
On your ME Day perform DLs, Cleans, Squats, Bench, Press, etc. Search message board "ME Black Box" for the details of this program.
Clearly, maintain size is largely a function of diet. The diet of many CF'ers is some form of the Zone Diet.
CF's program will increase your strength and speed, endurance, balance, agility, stamina, flexibility, power, coordination, and accuracy. that's it. it works.