A bigger anechoic chamber is not automatically a better one, and a smaller one to save money can cost you in accuracy. The usable space has to balance the object under test, the test distance and the low frequency performance. The wedges or absorptive walls take up part of the depth, and the true free field space is what remains once that lining is subtracted.
Low frequency performance is set mainly by the cut off frequency, which in turn is tied to the thickness of the absorptive structure. To hold free field behavior down to a lower frequency you need a deeper absorber and a larger room. So at the design stage the right cut off frequency and clear inside dimensions are decided from the frequency range of the product and the standard you test against.
Test distance matters just as much. Sound power and directivity measurements usually call for a set distance between the microphone and the source, with the microphone sitting inside the free field region. A room that is too small forces the measurement points close to the boundaries and lets reflection error creep in. When Anteck designs a laboratory, we run the acoustic calculation first and then fix the size against both the standard and the site, so accuracy is built in from the start.