fuel return leak

[kridz3120498]

I just got my first mustang a week ago and she has a gas leak coming from the return line. the owner of the lot I picked up the car from said it is just an o-ring but my local ford dealership only sells the ring that they think will fit in packs of 10 fore 40$. it is a 2$ o-ring and was wondering if anyone knew what size so i dont have to take it apart and then go get one. thanks.