There are a few things to consider when trying to determine the largest porcelain tile you can get. The size of the tile is determined by the size of the kiln that it was fired in, so the first thing to look at is the size of your kiln. Most home hobbyist kilns are only big enough to fire tiles that are about 12x12 or smaller. If you have a larger kiln, then you may be able to find some commercially made porcelain tiles that are up to 18x18.
The second thing to consider is the thickness of the tile. Porcelain tiles are usually between 3/8 and 1/2 thick. The thicker the tile, the more difficult it will be to cut and install. You may be able to find some thicker tiles at a specialty tile store, but they will likely be much more expensive than standard tiles.
Finally, you need to consider how many tiles you want in a single piece. Most people want their porcelain tile installation to look like one continuous piece, so they will try to find a single large tile that covers their entire floor or countertop space. However, this isn't always possible (or desirable). If you can't find a single large enough tile, you can always use multiple smaller tiles laid side-by-side or in an offset pattern.