Let your serving size be your guide. For puerh, the serving size is usually on the small side, because of the emphasis on multiple infusions. Measure your gong fu cup(s) and work your way backward.
My signature cup holds 110 ml or 3.7 oz, so, after allowing for the space the tea itself occupies in the pot, I like the 3.5 oz to 4.0 oz range (100 ml to 125 ml). This way I don't have to fool with a fairness cup or serving pitcher, and I can brew many infusions without getting waterlogged. Quantity really becomes as issue with those teas (notably puerh & dan cong) that I will sometimes push beyond 10 infusions. Even with my smallish cup, that's still well over a quart of liquid! A lot of people stick with even smaller sized cups and pots.
For guests, I tend to use the same size pot but shift down in size to the cute tiny cups in the 30 to 50 ml range (1 oz to 1.5 oz). For shu I have a 130 ml (4.4 oz) pot that requires me to shift up to a larger cup. I also have a couple of really dinky pots, 45 ml and 70 ml, that I use for either very small samples or really expensive stuff.
I just finally found a use for the enormous -- 11 oz -- pot that I got by accident: it takes the edge of my Assam teas.
Oh, and ditto to everything Hop said!