This could get interesting *;)
Domestic machines use all sorts of sizes from 45mm to 58mm
Commercial machines use 54mm to 58 mm, havnt seen a smaller one but I will no doubt be corrected *::)
Channelling issues smaller diameter PFs are easier to get close to level than the more open 58mm ones. So distribution and level tamps are less of a worry factor on smaller ones.
There may well be a reason some of the "modenr toy" machines from Krups, Breville Aldi *:o use smaller ones *is a cost and a scale thing. 58mm on a small machine would look wierd. Ignore them as most are ordinary to rubbish anyway.
Shot Quality is a consideration, it has been mooted in some circles that smaller portafilters give a sweeter result, this is a whole can of worms and worthy of its own thread I reckon. Della Corte and GS3s are some current examples of 54 or 55mm PFs I think and are some of the best machines in the world.
Some of the tiny diameter PFs are on 50-60s lever machines and were designed for smaller shot sizes than the generally acepted commercial volumes.
Why 58 and not 55 or 60 or ??? I suspect this "standard" goes back to the 50s or early 60s and has become a defacto standard. And I suspect some manufacturers have got into a rut of sorts rather than try new things (flame suit is on).
Apart from that I have been a happy user and owner of 51, 53 and 55mm PFs *8-)