The recipe asks for one 16 oz. package of precooked polenta, but I only have yellow corn grits (uncooked, from Bob's Red Mill - also labeled as polenta). How much do I cook for it to be equivalent?
Corn grits are not the best possible substitute for polenta:
A better substitute for uncooked polenta would be plain corn meal (1:1); for precooked polenta would be cornmeal that has been cooked into a thick porridge (1:1).
If you do substitute hominy type grits for plain polenta, your result will be somewhat different, but you can try 1:1, by volume or weight, either way.
Update: evidently some processors label their product "Grits" as well as "Traditional Italian Polenta". While I cannot endorese misusing the word grits, in this case, then, they are the same product so it isn't really a substitution, and 1:1 is the proper use.