Generally Saanens give the most....but they're not 'small'...and some Toggs give a lot, but again, they're not small. Obs and Alpines give a decent amount....as do La Manchas.
I like Nub milk best, and some of them will milk like a cow....but not all.
You could get a Saanen who makes no milk or a Nub who makes 15# per day...there are no set in stone rules, you have to buy MILK goats.
Genetics, genetics, genetics....if they don't have milk behind them, they will not put it in the bucket.
Feed to milk conversion will depend on quality of feed as much as genetics.
No such thing as a 'disease resistant' goat....you get more hybrid vigor w/ mutts, but ANY goat can get Tetanus, Listeriosis, Chlamydia, Entero...etc.
You have to start w/ good "milk" genetics, keep them healthy, vaccinate them, and feed them properly.
A great milk goat on junk feed won't milk....a junk milk goat on great feed will do better.
Why are you wanting a small goat? Is it because of ease of handling or space or what? I am asking because with my goats, some of the bigger ones are actually easier to handle than the smaller ones.