Average banner size is about 1200mm x 2400mm or 8' x 4' in old money, The finished article is usually a bit smaller after trimming and hemming etc.
If you are going to be hanging over an upper tier more often than not it is best to make them a bit narrower as they can hang over and obstruct the view of fans on the lower level.
Yeah the units it how many meters long you want it, i usually get 9 - 12 metres long at a time to have some 'in stock' for off the cuff painted banners.