is the 156 wide enough for a size 13...
is the 156 wide enough for a size 13 boot?
It is not. You need a wide specific snowboard.
I totally disagree. I have a 153 Jibstick I have ridden since last season with both size 11.5 and 12 boots. I ride about 24" stance with +15, -15 binding angles. Every boot and binding combo will be different, but seriously, you want some toe and heel overhang otherwise you will have zero leverage to lean on an edge when carving.
This board totally rips and the 156 will be fine for your size 13s.
It will be close with a 256 waist and a size 13 boot. If you ride a wide, ducked out stance you could probably get away with it, but you will probably get some drag when really laying into a turn on a steep slope. However, if you're gonna be riding mostly in the park you will be fine.