It depends how you like it to fit to be honest mate.
Lots of people go for a baggier jersey for increased chance of bounces and fashion etc.
I'm 5"8 12st and wear medium tshirts etc, but wouldn't buy anything smaller than a Large for paintball. I've had a medium jersey before, and while it 'fit', I didn't like it being just about big enough.
I usually wear a L/XL but it's just personal preference.
As you say, a size larger than normal tshirt size is a good rule of thumb.