This should actually be called a Shower BC (bioconverter), because, although there is excellent breakdown of nitrogen compounds and even organic compounds, there is little or no mechanical filtration.

Tubs can be any size. Have them before you build the frame. Keep in mind that the tubs will, at some point, need to be moved for maintenance, will be full of wet lava rock, and therefore somewhat heavy. You ideally want three rows high for maximum aeration, but two high would work if need be.

Spray bars can span tubs by length or width. The setup shown with the blue trays has spray bars arranged crossways like the .bmp schematic.



The one with the black trays spray bars run lengthwise. I have done both. Your call. The spray bars are 1 ¼" PVC (always use schedule 40 for all this stuff), should be about 6" apart, and will certainly keep all the lava rock wet with sufficient pump flow and splash. You'll need to surround the shower to:

a) avoid losing a lot of water;
b) avoid too big of a mess; and
c) avoid supercooling by wind chill.

Use a pump that free flows at least 1800 gph (MD18). I like the Danner mag drives from Aquatic Eco Systems, Inc. at about 2400 gph (the MD24). I have even used a 3600 gph (MD36) on my shower at home, if you have an immense fish load or on a bigger scale.

www.aquaticeco.com/subcategories/2322/Supreme-Mag-Drive-Pumps-by-Danner/

Anyway, here's a bunch of pix with tape measure for scale. The spray bar holes are ¼". The tray drain holes are 5/16" and more numerous. You want to avoid any water (and sludge) accumulation in the trays, do drill at least some holes in the low spots.