Having worked in the sign industry for almost a decade, and having been in multiple bands over the years, I can give some insight.
It seems as though most bands think bigger is better, until they actually see the end result, and have to deal with transporting/ maintaining the banner. A good rule of thumb to keep in mind is that 1" of letter height = 10' of maximum impact viewing distance. That's maximum impact, not total viewing distance. Being that a lot of bands play small bars, this translates into a much smaller and more manageable banner than what they may end up with. A banner 2' tall by 5-6' wide is more than enough to get the point across.
When it comes to designing a banner or logo, most sign shops can do the work, and usually charge much less than a design firm. Granted, you may have to spend a little more time to get the desired end result, but if money is a concern, it's a viable option.
Someone above hit the nail on the head when it comes to vinyl/PVC banners- NEVER FOLD THEM!!! Always roll them up!!! I can't tell you how many time people would bring a banner in for a date-change or name-change, and the thing was almost in shambles because they thought it'd be a good idea to fold it! Best thing to do is buy some PVC pipe from Lowe's or Home Depot, add a couple ends to it, and put the banner it there when it's not in use. It'll extend the life of the banner almost indefinitely, especially if you keep it clean and out of direct sunlight, when possible.
Most sign companies offer 2 types of banners- vinyl-lettered and printed. Both have their pros and cons. Vinyl-lettered banners are inexpensive, and work great if your band logo is really simple... mainly words, or words and a simple graphic. The downside is that after several years, the vinyl (especially calendered) tends to shrink, leaving the adhesive behind. Makes the banner look crappy. Leaving it in direct sunlight will speed this up considerably.
Printed banners are pricier, but allow you to put way more on the banner than vinyl alone. If you have a complex logo, with a very distorted font, shadows, photos, gradients, etc., printing is the way to go. However, just like vinyl, printing has a finite life, and will start to fade after several years. Again, direct sunlight will affect this process as well.
Besides vinyl/PVC, banners can be made with a mesh material (as noted above) which for larget banners can reduce the weight considerably. The trade-off is that the image will be somewhat muted, as there is less surface area for the image to be printed to. Another material is nylon, which is slightly lighter than PVC, but looks great, and is easy to maintain. If it's a printed banner on nylon, you can fold the banner without doing any damage in the long run.
