I don't think you'll lost significant amounts of actual muscle tissue over a month layoff. You'll feel and look smaller and weigh less because of a loss of glycogen and water weight but don't freak out about it. Your size should rebound within 1-2 workouts. Your strength should similarly rebound after a little while, because when you don't lift your nervous system "unlearns" the movments and becomes less efficient at it.
Moral of the story: don't worry about it. Just make sure to eat at least maintainance.
-casualbb