When I started out 2 years ago, the HBS guy advised me that my old beer fermenter was too small and I must buy a new 30 litre one for adequate headspace.
On the weekend I decided to do 2 ferments since I was given a large amount of dextrose, (I usually use sucrose-white sugar). I had a hunch that my old 25 litre (or so) fermenter would be OK, if a little full. So I put the 8kg of white sugar in the small fermenter and started a double run of Alcotec 48. After 3 days the activity is starting to slow, at 25 degrees, and there has been no overflow or problem. I expect after the usual 5 days it will be fine.
With beer fermenting, I can see that this is more critical since beer worts are foamy. Sugar washes however have not much stuff that allow emulsions to form and consequently don't get very foamy, despite the tremendous acivity of a turbo yeast.