My 07 mazda3 I Sport came with 15" wheels, I would like to upgrade to 17's. I read somewhere that this particular trim model will not accept anything larger than
16"(something about the brakes). Can anyone confirm?

Yeah you can usually change to a bigger tire size as long as it is 15
Like 225 60 15 to 225 70 15
But going bigger would very likely be too large for the wheel well and make it hard or even impossible to make turns.
I am not even sure changing from 15 to 17 would be possible at all.

Thanks for responding. The reason I was considering changing to a larger wheel (not just tire) was to eliminate the canyon I have in my wheel well. There definitely is some playing room, I just don't know if it is possible from a mechanical/driveability standpoint.