If the mouse isn't bigger then the widest part of your snake your perfectly fine. I was nervous when I first fed my Ball Python an Adult Mouse but he took it easily. I thought he wasn't going to get it down but he did and he kept it down. It will take a little longer for him to swallow but thats Normal cause the prey is bigger then what the snake is use to. Just try not to be nervous, also something you should know if your feeding Live is that Adult Mice don't die as fast as Hoppers so don't get worried if the mouse is still alive 20-30 seconds after he wraps around it. =)
Good Luck. =-)