Ideally, the membrane should be three times the width of your tile. Using 6" tile, then your membrane should be 18" wide.
In a perfect world (and very few are) you'd also install a soft joint on each side of the crack. "Hard joints" are your standard portland-based grout mixture, a "soft joint" would be done using a sanded caulk. That's right, a sanded caulk. Because of the requirement for a sanded caulk on a floor, you can see why so few worlds are perfect.
You really do need to get a tech sheet from the person who sold you the membrane because not all membranes are installed using the same methods. If they don't have one, find out who manufactured the membrane and go to the manufacturer's website. Most have that stuff on-line these days.
And, as you know, there's not much sense in doing it if you're not going to do it right. Using the membrane is a step in the right direction. Using the membrane correctly is the next. Get either the membrane info or a membrane refund from the tile shop.