Good advice from Rip and Mosca.
Usually 20lb Dacron or Micron is used for backing (sold by Cortland, Scientific Anglers etc).
If you don't have a fly shop nearby, or want to do it yourself, take a look at the packaging the reel came in if you still have it, or post the make and model here--- maybe some folks have the same one and can tell you how much it will take with a 7 weight line.
If worse comes to worse, 100 yards should be plenty for a typical 7 weight reel. You won't need it all for bass- in fact you probably won't see it when you fish, but it will take up room on the reel so the fly line isn't tightly coiled around a small diameter on the spool. But depending on the reel it may be too much, which will make your fly line jam against the top of the reel. To get the right amount on without knowing the reel's backing capacity, it's a little time consuming, but one way to figure it out is to put the fly line on the reel first (don't tie any knots yet), put backing on top of the fly line and reel it in until you fill the spool- and cut the backing. Then take it all off and put the backing on and the fly line on top of it. Here's how to do it:
1. If you have a weight forward taper fly line, mark the back end of the fly line with masking tape or a permanent marker. This will help you keep track so you don't put the fly line on backwards. (You want to make sure the thicker head ends up coming off the reel first when you're all done.)
2. Without tying any knots, wind all the fly line onto the reel.
3. Again with out tying any knots, wind the backing on to the spool until you're about 1/3" or so from the rim. Leaving some space and not over filling the reel will make your life easier when you're fishing. If it's over filled, the fly line may build up on one side and become jammed against one of the pillars that holds the two sides of the reel together. Cut the backing.
4. Take the backing off the reel.
5.Take the fly line off the reel
6. Tie the backing to the reel spool with an Arbor Knot You can find instructions for many popular fishing knots here: http://www.animatedknots.com/indexfi...matedknots.com
7. Tie the back end of the fly line to the backing with a Nail Knot
8. Reel in the fly line. The front of the fly line should end up on the top of the reel spool.
9. At this point all you need is a leader on the front of the fly line attach with your favorite connection (nail knot, loop to loop etc). Add piece of tippet to the front of the leader and a fly and you're good to go.
Again, if you don't have a shop nearby and don't have the packaging, post the reel make and model here. Hopefully folks can chime in with a ball park amount of backing to use so you don't have to go through all this...