That size fermenter is designed for 2.5 gallon final batch sizes. The extra 1.5 gallons is for krausen head space. You can probably do 3 gallons fine in there but there's a greater chance of krausen coming out of the airlock since there will be less head space. Should be fine for most ales but wheat beer yeasts foam up a lot and need more headspace on average.
The Stainless Bucket Fermentor is the perfect fermentation vessel for 2.5 to 3 gallon batches. The small design enables it to easily fit inside a refrigerated unit.
I have one and have made a two gallon, a two and a half gallon, and now a three gallon batch in it. I picked up a stainless barb and some silicon tubing for blow off just in case(very similar to Anvil's blow off kit). So far, no overflow, but I haven't done a wheat beer in it. I like the fermentor quite a lot. Small footprint, easy to clean, and the rotating racking arm works well.