As Brewbama has said, you will be OK. I brewed a RIS last Sunday and always put a blowoff tube in my 7.5 gallon conical fermenter I had a headspace of 2 gallons and the beer still blew some yeast through the blowoff. i am used to this with a big beer so I take steps to avoid the mess.
If you are using a blowoff in a bucket or a carboy, make sure you put the blowoff jar in a bucket since you will probably fill the jar with yeast and beer, depending on its size. I had a mess once before I got the conical and learned the hard way. Also, make sure to check the blowoff tube from time to time since it could become plugged with the krausen and pop out the stopper/blowoff tube from the fermenter.