50 amp double pole breaker, direct burial wire (6-3 or 4-3 see below) 6-3 UF is more available locally. 2 feet underground no conduit needed, although if you want to protect the wire you can use PVC conduit and dig less deep. Or rigid conduit if you cant dig deep.

http://www.doityourself.com/stry/calculating-how-deep-to-bury-outdoor-electrical-wire

And look at voltage drop for that distance using 6 gauge. At 150 feet I calculate 5.9 volts dropped. At 120v that just meets the 5% maximum for voltage drop. You might want to switch to 4-3 for that distance. Would probably have to get that at a electric supply co or online. Otherwise, 4 gauge THHN/THWN conductors in conduit

voltage drop equation

wire resistances per 1000 ft

Also need a new ground rod placed and bonded to at the new sub location.

Note that's Sched 80 if you use PVC conduit for shallower cover. Other than that Vith has a very nice analysis.

I figure to hook it up to the 200 amp main at the pole

Do you mean that you'll add a 50 amp breaker to the box that contains the main at the pole? Because the breaker has to protect the wire that's downstream, so if you are actually planning on attaching it to a 200 amp breaker then the wire has to be sized for 200 amps.

I never direct bury electrical wire. It's too easy to put a shovel through it 5 years later when you forget exactly where it's sitting. That's going to be an expensive chunk of wire, and I'd protect it with conduit.

so you think I should attach it to a 50 amp breaker at the pole and then run it tgo a 50 amp indoor box?

www.dbelectric.us

"so you think I should attach it to a 50 amp breaker at the pole and then run it to a 50 amp indoor box?"

Yeah, pretty much. You can install an additional 50-amp breaker at the subpanel.

At that distance, you may have to think about an additional ground rod at the out building.

He needs a disconnect of some sort at the outbuilding (a main breaker in the panel is one way of providing that).

He needs a disconnect of some sort at the outbuilding

Isn't there a "6 throws" rule or something where if you have less than 6 breakers that you'd have to flip to turn off all the power, then that's good enough?

Either way, I'd still suggest a main breaker but I'm not sure it's totally necessary by code.