I've done it up to 11x14, which is the largest size I (rarely) print anyway.
The trick to it is that you need to keep even heating. It's actually really quite easy- move in circles. Start by tacking down the middle, rotating the iron in an area roughly the size of a quarter, then move out in bigger circles. As you reach a big enough circle, lay the iron flat and move in slow circles. This way you evenly heat the whole piece, and tack down all of it. However, you MUST put a sheet of blank paper (regular printer paper or two sheet of newspaper work fine) between the photo and the iron, otherwise the emulsion bubbles up.
So really, just be sensible about it, and it's quite possible. I learned to do this proficiently after just a few prints- mostly because I don't have access to a press.
Keep the iron on a low setting, I put it on silk setting (level 2 to 3 out of like 9), too hot and the photo bubbles; too cold and the sheet won't adhere properly.
As I said, I cannot attest to larger print sizes; I'm certain the larger print would be more difficult to keep heated evenly.
Happy mounting!