I ran 215/65r16 BFG ko2s at both stock ride height and with a 2-in lift kit on. At stock the rears would only rub slightly if you got near bottoming out the rear suspension, otherwise it was fine. I had no issues in the front. That's a 27-in tire BTW and wheels matching the factory offset.
I'm currently running 235/75r15 Mickey Thompson Baja Boss A/Ts (29in diameter) Tires rub like a mofo front and rear even with my lift kit. I kind of expected it to be an issue so I installed the Crosstrek struts and springs that I acquired super cheap a while back, still rubbing. I ordered some Rallitek 1-in lift springs for the front and 1 in overload lift springs for the rear (I frequently have two or three adults riding in the back during Uber rides so I figured the overload springs would be helpful when carrying the extra mass). Just installed the rear springs last night and all is well back there. Currently waiting on the front springs to get manufactured. Going to order Crosstrek subframe mounts to help give me even more wheel well clearance as well as help with alignment.
So what's the largest tire you can fit on an impreza? How much are you willing to compromise and how much are you willing to cut? If you keep in mind the speedometer inaccuracy and are willing to accept that then I'd say go for 27in diameter max. Anything larger and you're talking major modifications.
29s on an Impreza is stupid, but I love it and I'm willing to accept the compromises.
Don't worry I'll post pics later 😉