As much of the Martian surface is covered in rock fields - on average, there is one significantly-sized rock per square meter that could be an obstruction hazard - the team needed to find the optimal size of a ball that can be efficient at catching the breeze, light enough to actually move and, fundamentally, big enough to roll over the rocks without getting stuck. It turns out that this optimal size is six meters (20 ft) in diameter.
"We found that, in general, the larger the diameter, and the lower the overall weight, the better the rover performs," Mazzoleni in the NCSU press release.
Another significant finding was that any wind-blown tumbleweed design would be in for a bumpy ride during their mission. Martian gravity is only one-third that of Earth's and the planet's atmosphere is only one-hundredth the pressure that we experience at sea-level - which means it takes a small bounce to get these things airborne. In fact, they will likely do more bouncing than actual rolling, according to the simulations.