I have recently bought a Burton Custom...
I have recently bought a Burton Custom V-Rocker 156 and I am looking at getting a Wheelie Gig bag, is the 156 size big enough to fit a 156 board or is it too snug?
No, it has more than enough room. Don't worry about that at all. It may be wise to size up on the board bag incase you ever get a bigger board to go slay some powder.
Get a bigger bag... You'll thank yourself later...