Hi, I was on the same quest no too long ago. The length is going to be something you have to determine based on your shooting style and needs. For example, if you're shooting kids or models sitting on the floor you may want the extra length. You can lower the height and extend the length, but if someone is standing in the same scene it may become a hassle. Also, if you plan to drape the backdrop over furniture etc, you may want the extra length. So, you need to ask yourself questions like these.
Here's the path I took (money being an influence): I started with paper rolls; a couple of 4' x 20'+ and 9' x 20'+ . Available from any of the big photo stores. These work great and they're cheap. The problem, they're a PIA to go anywhere with. If you ever want to incorporate texture in your background you can't ( at least using the backdrop). If you have creases and you don't notice, they stick out sorely in prints being that the rest is seamless and flat.
Later, I went to a local fabric store and bought some duck cloth canvass. This is a nice alternative; solid rich colors, seamless, and thick fabric. Similar characteristics as paper, but more portable. The limitations here are: width ( about 4' or 5' by any length); though not a big deal if it fills your frame. Plus, if you don't need the length you can just use it horizontally. Another downside is creasing caused by folding the canvass. Of course you can iron this out.
Finally, I decided I would to just go ahead and invest in the typical muslin backdrops. Mind you, this is the same material available in fabric stores with the exception of size and dyed patterns, for a few dollars a yard. But, since its for photography the price is ten fold. The advantage of using muslin is that the possibilities are endless; you can incorporate texture, its very portable, you can drape it, and you dont have to worry about creases. The caveat is that its difficult to get the same flat/solid appearance of paper or canvass. A place you may want to check out for muslin backdrops, cost being a factor, is Owens Originals( owens-originals.com ) . No affiliation, but they seem to have decent quality to cost ratio.
Hope this helps. Good luck.