I have a stock 350ci what i the minimum size exhaust pipe if its 2 into 1

Keep in mind, when customizing your exhaust, the main goal is to improve exhaust flow, not restrict it. Reducing the exhaust to 2 1/4" at the cherry bomb will affect your exhaust flow. Will it work, yes. Will it decrease exhaust flow and performance, yes.

good point

that muffler cme of a 350 but it was dual exhaust, Im piped 2 in 1

Yup, so off of the exhaust manifolds, you have one side that crosses over into a "Y" pipe that joins into a single exhaust. I would highly recommend staying with the 3" all the way back.