In my experience the best bass guitar isolation rooms start at 28' X 14' 10" with 12' pent roof. I have tried very hard to achieve good results in small isolation rooms, even ones with most of what spare area there is taken up with bass traps, have been muddy and excessively resonant in the low mid on the microphone sound.
If you're close mic'ing with a directional mic it shouldn't matter a lick what size the room is unless you're struggling with proximity effects.
