The guys at HD
suggested 8/3 with ground. My question is what size circuit breaker should I
have at the house panel,

8ga Romex is good for 40a. THHN in conduit is good for 50a if you have 75
degree terminals in your sub-panel and breaker feeding it. (most do when you
get over 30a)
For a few more bucks you could go with 6 ga and have some extra capacity but I
doubt you need it unless you expect to have more than one person running tools
.... or a heater.
If you are looking at electric heat, 80a may be more in line.
Your sub-panel will probably have 100a busses and the breakers are pretty much
the same price in these ranges so it is really just the price of the wire.
Look back a few weeks in these electric threads here and there will be more
than you want to know about 4 wire feeders and whether you need supplimental
grounding rods. Supplimental grounding and disconnect issues only come up if
this is a separate building.