If you look at Donald's Senior Bowl weigh in measurements, he's not that far off. He was a shade under 6'1" (6' 0 7/8"), but his 31 3/4" arms aren't really short for his height.
An interesting comparison is Donald vs. Washington State safety (yes, that's right) Deone Buchannon, who is lauded for his long frame and wingspan:
- Donald: 6' 0 7/8" 288#, 31 3/4" arms, 9 7/8" hands, 77 3/8" wingspan
- Buchannon: 6' 0 7/8" 216#, 31 3/4" arms, 9 1/2" hands, 78" wingspan
Except for the 72# weight difference they are uncannily similar, yet one guy is "long and lanky" and the other "short and stubby". Obviously Donald's game isn't going to be playing JJ Swat and trying to bat down a lot of balls, but I'm not sure why he couldn't be effective. Justin Smith was 267# when he came out of Missouri in 2001, and he hasn't had much of an issue with durability despite playing DT and 3-4 DE at 285#. Luke Smith, Donald obviously has exceptional core strength, and he uses his hands extremely well.
If Armond Armstead actually turns out to be what we hoped he would be (obviously not a given right now), one could do a lot of creative things with a 5 man core DL rotation of Rob Ninkovich, Chandler Jones, Linval Joseph and Donald and Armstead. You could run a basic front 4 of Ninkovich-Joseph-Donald-Jones, bring Armstead in instead of Joseph on rushing downs or instead of Ninkovich if you wanted to have more of a 4-3 under or 3-4 look with Jones as the Leo, confuse people by switching Armstead inside and Donald outside, sometimes move Jones inside with Collins or someone like Marcus Smith or Christian Jones playing a speed rushing role, use Hightower on stunts, .... The options are pretty extensive for an imaginative DC. I'm not sure Matt Patricia is that guy, though he seemed to be doing better early this year before injuries took their toll. But there could be some very intriguing possibilities.