As big as you can afford. No one has ever said "gee I brought to much ammo to this gunfight" or "damn, I can't believe I had too much fire extinguisher." Consider a portable 5 gallon foam extinguisher too, powder is good, but for most of home burnables foam is way better.
You know that sound as you portable car fire extinguisher runs out? No, not that fading fwwwttt sound, but that steady humming... of the fuel pump still running, pouring fuel out of the melted fuel line, down the block, over the hot exhaust and puddling around the front tire. I nearly had those people's car put out, no visible flame, I just turned and walked away, about 50ft behind me the tire blew up. Car burned to the ground.