I have a 5 cu ft that fits my 6 gallon better bottle with airlock fine. In hindsight, I wish I had gone bigger so I could do multiple batches at a time with similar fermentation temps.
I may still go bigger and make the 5 cu ft one a lagering fridge someday in the future.
Seriously think about what you want to do or may want to do in the future and get there right away.