I've upgraded to 19s but with 235/35F and 265/30R tires so it's slightly bigger in diameter now. Will my spare tire still be ok to use for the rears?

Want to know as well....

As long as your overall diameter of wheel+tire is the same then you should be fine.

It's not that's why I asked. The closest tire size to match stock is 225/35F and 255/30R. But those tires would've had a slightly stretched looked so I opted for more rubber. But I'm not sure if the small difference would cause any issue. Specifically for the rears.

If you have 4 matic then technically the answer is no

You'll over wear your differential

I would suggest you only use the spare on the non drive axle side (i.e. use on front for rwd and rear on fwd). I would also assume you'd run into trouble with stability and traction control. I've heard that there's a small tolerance for diameter difference like no more than 1/4th to 1/2 an inch but I could be wrong and wouldn't bet on it unless someone else can confirm.

If you increased your rim size by 1 inch and dropped your tire size down 1 side wall height that would be the same rolling diameter. The width does not matter.

If he were using it to commute daily I'd worry. (I know some idiots that leave spares on for weeks and months.) If you use a spare correctly for the time it takes to get to a shop I doubt you have much to worry about in terms of excess wear.