Choosing hyperbaric chamber size involves knowing if you want more space to move around in (diameter size) and the length of the chamber. If 2 people are going to use the chamber most people prefer a 10 foot chamber.
Note: 10 footers are not factory standards and usually have to be made to order. Ask to see if there are any 10 footers in stock if you don't want the standard 7 to 8 foot chamber.
A 23 inch hyperbaric chambers gives enough room to lay down in and you can't turn around or sit up. These are the most affordable sizes. The types with just one window are even the cheapest models. These feel very claustrophobic for most people. You can bring a book and read in there but it feels very cramped. These also get hotter so I find 2 air compressors helpful in giving more air turns. The larger chambers have more surface area and allows more heat to escape.
The 28 inch hyperbaric chambers are the most popular size if its a 1 person use. These have enough room to move around in and not feel claustrophobic for most people. These have about 70 square foot of material while the 23 inch chamber has about 40 inches. There are some 28 inch chambers that have only one window and those are much cheaper than the ones with 3 windows.
32 to 33 inch chambers
These are the chambers that are the best fit for 2 people going in the chamber and for both people who don't want to feel claustrophobic. These hyperbaric chambers usually have the most windows. These usually only go to 1.3 ATA as it is much harder to reinforce a chamber to go to 1.4 or 1.5 ATA when they are this large. Most people buying a really large chamber and pay a premium for this size need a larger size and usually plan on using the chamber longer than temporary use. Those that had injuries usually get the 23 inch until they get better and sell it.
40 to 46 inch chambers
These are big enough for 3 people. 2 adults and a child can stay comfortably in this size, especially the 46 inch which is the biggest size on the market (see Grand Dive PRO). This is almost 4 foot high and at 10 foot long it gives people who really want room all the room they need.
Note: There are only a few chambers that go to 1.4 to 1.5 ATA. All chambers that are rated for higher pressures over 1.3 ATA MUST HAVE STRAPS.
The most popular higher pressure large size chamber is the Oxyhealth Quamvis 320. This is a large size like the Oxyhealth Vitaeris 320 but that one does not have straps.
The Oxynova 720 is Oxynova's largest chamber and is 28 inches. This is the only chamber that is an exception and does not have straps. It has a Teflon coating that reinforces it. For those that don't want straps and want to go to 1.4 ATA this chamber costs nearly as much as the 33 and 40 inch chambers that have straps that go to 1.5 ATA.