This excerpt from the tournament help page seems to indicate that you'd have no penalty.
How do I withdraw from a tournament?
You are allowed to withdraw from a tournament at any time using the Withdraw link located on the right hand side of any tournament homepage. However, withdrawing from a tournament while you have games in progress will automatically cause you to forfeit all your games on time and negatively affects your timeout % ratio. If your timeout ratio is too high, you will not be allowed to play in future tournaments.
I am in this big tournament of 100 players and i want to leave it..I got no games running in that tournament right now..can i safely withdraw from it without it affecting my rating?
Thanks once again
FF