You should have gotten an order packet with additional information. You can order larger ones and the prices should be in that information. If you do not have it you can call DPS. I ordered a 16 x 20 of ours and just went with the enlargement, not the mounted portrait (at their recommendation to save $$) and it was $80 or $85, two years ago. DPS # is 407-827-5099.
You can also purchase the CD of the images, but I think it is still $1000. Someone correct me if that has changed. That's what it was when we had ours done, so we are waiting for the three year mark to get it at a reduced price. Keep in mind that they only preserve your images for five years, so you have to order it within that time frame or they will no longer have them.