CFM stand for cubic feet per minute.

The size of your cab is 22.2 cubic feet of space.

180cfm would circulate the air in the box around 8 times per minute depending on location inside the box.
now it would all depend on what type of light you are using. I am guessing that 250watt means 250 watt HPS. Right?

a 256 would be a better idea to me. Better safe then sorry and if you don't already have the 180cfm fan then it wold be better to not waste money and have to buy another if the 180 didn't work for you.

the 180 should work but as the plants grow and fill up space you would notice an increase in temp so the more the better a 256 would be fine. 180 might be though.

final answer. get the 256cfm.