the potency varies greatly from cacti to cacti....natives say a length of san pedro the same size as your forearm is a good dose.
and about smoking....i've only ever smoked powdered green flesh (mescal containing portion), to great effect. it doesn't take much of a psychoactive chemical to reach effect when smoking if the chemial can survive the heat from smoking (which mescaline can).
with whole cacti...one POWERFUL TIP that i've learned is this.....
take a knife and nick the cactus in a few different spots (on an 18" probably a good 10 nicks), jut poke the tip of the knife through a little bit. then leave the cactus in the dark for a week or two (check it for mold, but leave it in the dark.) after a week or two (i think i did 1 1/2 wk) take the cactus, cut it horizontally into stars about 1/2 in. thick. leave these stars to dry in the sun, or in an oven (but i don't know what temp, you'd have to look it up).
this process will 2x-3x increase potency. since the cactus produces mescaline as a bug repellant, the poking and the leaving in the dark (since in time it will soften in dark, increasing bug risk) will signal the cactus to increase the mescaline content.
when i did this technique with 2- 6" peruvian torch cactus, the result was about20 cactus stars, and 2 of these was an intensely visual experience. i just made a tea of rosebuds (more vit. c than oranges) and cactus stars to rehydrate the cactus, then i chewed down the gummy stars (like candy...). and of course drink the tea as well.