Plant's are a much better, less sensitive filtration system.
Filters are more for mechanical removal, water movement, some bacterial back up, do not play a large role in plant tanks, as much as fish only systems which are what FBF are designed for, NH4=> NO2=>NO3.
FBF crash if they are left off for 15 minutes or more, these bacteria need a lot of O2 if they are processing larger amounts of waste.
Add plants to your filter if you need more removal.
In answer to your question, about 12"x2"dia and 10-16 oz of sand will work. But you may wish to ask the other questions before doing this.
I totally agree with Tom. I used to use one about 8 years ago when I was really into discus. I bought all kind of wet/dry filters including fluidize bed. IMO, it's a pain to use. I wouldn't say 15 minutes pause but an hour hour pause will kill all the bacteria inside there and it smells awful right after. The water flow needs to be perfect. Too powerful of water flow will blow most of the sands out. Too weak will not make it an efficient bio filter. This fluidize bed filter was out to beat the bulky wet/dry system but failed to do so. If you have plants, there is no need of anymore bio filtration device.