Thinking about upgrading my refugium.

Currently I have a 70 Gallon sump(total dimensions) on a 230 gallon DT.
My refugium section is actual operating volume about 13 gallons.
Just doesn't seem like that would be enough. My chaeto is still only about a gallon or two and growing. So not sure if it's effective much yet. I only have 5 fish and only one is bigger Naso.. others are tiny.
But in the future my bio load will of course get bigger so I am considering a through the wall to a remote refugium.
Probably about 50-65 gallons would fit in the space.. first question is if it's even necessary to build it deeper since I can only have X amount of surface ( about 30X36)area that the light can penetrate. I am thinking about 12 -16 inches deep. How deep is too deep for chaeto to grow?
Is this a must do since my current refuge is so small?
Thanks