Warband are based on a rough square or oblong between three and six ranks deep.
But frankly if you want to present them as a mass of warriors you can. All you really need to do is make sure you can differentiate them from units in skirmish order (bases 1-2 inches (1-2 cms) for individual based models, or 1 base width for element based figures).
That is all you really need.
Warbands can be any size from tiny to large, as is the case with all units. The size of the unit depends on your scenario or research army lists.
Size depends on what you decide as the size of a average unit.
Thus if you average unit is 24 figures, a tiny unit is 5-6, a small 12-16, and a large one 36 to 40 figures.
If your average unit is 12 figures, than a tiny unit is 1-2 figures, a small one 6-8 figures, and a large one 24-36 figures. etc
There are rules in the book on how to calculate these.