3 or 4 inch main line

I made my bathroom bigger and will be relocating all fixtures.I picked up a 1.6 gpf toilet and would like to know is 3 inch pvc big enough for the main line to the sewer.Would 3 inch have a benefit over 4 inch.See originally I planned on 4 inch but was told by a friend that 3 is plenty big enough.Now I did pick up a closet flange for 4 inch and outside where the pipe goes in it measures 5 inch to accept the 4 inch pipe figuring the pipe is 4 1/2 outside.Now I already cut my 5 inch hole to place the flange in so that it rest on the floor.Now if I took it back and picked up a flange for 3 inch pipe would the the screw holes for screwing to the floor be far enough apart so that it would fall outside my 5 inch hole.Wondering if I'm stuck using the 4 or if 3 would still be able to screw down.Other then a small amount of cash I might save would I be better off with 3 inch.Also the orginal cast pipe appears to be 4 inch inside.I will be cutting this out and using a fernco fitting to join the two.Do they make one to go from 3 to 4 inch. Sorry so long

4" is overkill for your situation. Back in the day 4" was the norm for homes all the way up to the roof. In KY you must increase to 4" before passing through the building wall.

The only time you need 4" to a toilet in my area is when there are 4 toilets in the home, which is dictated by the DFU load on 3". (36 DFU's)

pipe

You have two different situations. The 3" main is adequate as long as you only have 3 or fewer toilets in the house and one of them is at least 15' from the other two. I ALWAYS use what is called a 4x3 closet bend for my toilets. If you use that then your 4" flange will attach to the riser and the 3" will connect to your vent fitting before going to the main.