The amount of hardware cloth you need depends on how big of a stand you're building. You going to go ahead and do a two hive stand? Or larger even?
The hardware cloth comes in three sizes that I've seen - 2 foot, 3 foot, and 4 foot. Two and three foot are the most common. My hives are 20"x18", so the two foot wide is plenty. Then it just depends on how big of a stand. The two hive stand needed two pieces no smaller than 20"x18", so a length of 3 feet was enough.
When I made the large brood box hive, I also used a piece that was 2'x3'.