One way of doing it by using the original negs - take them to a real pro, who may be able to create an "interneg" - this is basically a standard enlargement, but made on positive film instead of paper. A negative "printed" on to positive film makes a new, large negative which can be enlarged again. The process does lose a little clarity, but not as much as over-enlarging from an original or from an existing print. It is, however, EXTREMELY expensive.

Ask about locally, but NOT in your "average" processor. Jessops might know, your local paper might know or a pro wedding photog might know.
However, it is always advisable to look at the photo VERY critically, to ensure that all the factors are excellent - sharpness, lighting, etc. - if there are any faults, they will look a lot worse when magnified.

Thanks for one bit of info, though - never having used APS, I wasn't aware that there were enlargement restrictions on it - I use 35mm and have managed to get a 20" x 30" out of one of those, without significant degradation. Mind you, it was Kodak Ektar 25, which is nearly grain-free

HTH
Snorri
Twilight Zone paparazzo