What size is the exhaust tip?

Engine shouldn’t be idling for extended period. Unless it’s really cold and you have heater on high. Prius will only run the engine when it’s needed. So just first check that there is actually need for modifications. Sorry, no help for diameter but it should only take seconds to measure it with dial caliber or even a tape meansure.

 

Ventilation air intake is under the windscreen and wipers. I doubt that a little longer exhaust would be much better. Except you aren't using the ventilation. Then the gases can enter the cabin through the ventilation air outtakes behind the bumper (at the side of the trunk). Instead of a longer exhaust pipe, I think, using the ventilation (for forcing the air direction) would be more effective, perhaps combined with an air filter made of active-coal.

SM-G900F ?

 

However the outer diameter of mine is about 43 mm / 1,7".
But it's not horizontal but diagonal downwards a little. And making is longer will reduce the space to the street. So better take a look first.

SM-G900F ?

 

I guess if you crank the heat high enough the car's computers will force it to keep running most of the time? So I'd guess whats happening, is your sitting long enough the fumes from the exhaust are drifting around to the cabin air intake, at the bottom of the windshield? I can't see an exhaust tip extension helping with that.

Personally I don't like any amount of engine idling while just sitting, either AC in summer, or heating in winter. Some places it's illegal to idle more than a minute, never seen it enforced though.

What area in Canada?