IF my exhaust pipe is 2 1/2 outlet what size does my tip inlet need to be.

2 1/2 or 2 1/4

I know its kinda stupid sorry....

 

A tip made for 2.5" outlets should already be expanded to fit over your pipe. As you go back in the exhaust system the previous pipe should always slip into the next one in line (when using slip fit pipe and clamps) This helps prevent leaks in the system and reduces corrosion inside the pipes, since there isn't an edge to be erroded by the gasses and hold water.