I did not experiment with the size of them. What I did was check absorption coefficients of everything, the carpet and plywood included.
It is rather interesting how I came about and designed my HT and first I have to explain a few details before how I made the Helmholtz Resonator.
The way I found my HT being built was I had left my apartment for a weekend vacation. When I arrived at the lake house at night, the usual automatic motion sencing light did not come on, so I stoped, walked back up the steps and turned on my headlights to see where I was going. There was a large hole in the ground dug up over 20ft deep with rebarb sticking out the ground right where I was about to walk into. I called my folks immediatly in distress and exclaimed that I was nearly impailed by rebarb, and demanded a good explanation. They explained they were having a storage area being built and a garage. By the following week I had plans for the 20' X 20' area to be a HT, and we made up the design as we went, also adding a bedroom above the garage.
I began design on the HT by first placing a subwoofer in the room near where I thought most appropriate. This was a DIY subwoofer I had constructed while the bedroom was being built. What I then did was place the subwoofer in the room with a radio connected via headphone jack, and I walked around the room. The room was entirely concrete and steel, with no framing. I played ace of bass on my radio at volume level 4, and walked around the room standing in locations that had dips or peaks, and moving my head in and out of these areas observing what they sounded like. When I was at the rear of the room there was an espicially large area on the rear wall from which the air pressure would actually breeze past me in a woosh kind of tone/feeling. The area next to it was a corner that had a light feeling gentle rising type of bass response. If you have read the REW thread, this is where two of the 12" woofers are now located. I designed my room size around this taking oblique room modes into high account for even modal distribution, and left areas suitalbe for room treatments within the given space.
To size the Helmholtz Resonator I took the axial and oblique modes of near 80Hz into careful consideration after doing the room mode calculators. I refered to an online caulcultor for caclulating the sabines for RT60 in several scenereos, changing room size and factoring that I did not want to live, or to dead a room. Using an online Helmholtz Resonator calculator, I tuned the size of the box for an effective 80Hz absorption, and used the vertical size based on the listening height of the rear row. I did not have excel to calculate the holes size, so I used the required size of a single hole, and spread this over an area I thought seemed appropriate. I wanted it to be effective at lower frequencies, so I used large holes. I attempted to do my best to make the coverage of these holes as best for broadband bass absorption, without overdoing it. Basically I took what knowledge of hearing I had in my previous HT, everything I had learned from people I consider experts such as Bryan or Ethan and applied it.
To do that I did not spend hours over the internet. I used what knowledge I had and contemplated each idea for many hours while simply sitting in my HT area, listening to my radio, and occassonally enjoying a few cold ones. A good environment for me to me design is a stress free one. I did spend a good week checking facts and doing calculations online.
I asked for some help designing the Helmholtz Resonator in an acoustics forum (not here), asking for a member with exel to do the calculations for me, but there was no reply. I'm happy it turned out so well.