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Hemiduty,
Welcome to IIS!
The biggest amateur scopes that I know about are at the 30" mark. As Paul has indicated, two of these were manufactured by SDM telescopes, with the owners of both SDM #1 and SDM #2 members of the forum.
OMI Torus, a US mirror making company, has just installed a 30" mirror grinding set-up because this size is beginning to grow in popularity. There is probably less than half a dozen scopes of this size in Australia. There is probably another 30 or so amateur-owned scopes in the 20"-25" size in Australia, mostly made by Obsession Telescopes of Winsconsin USA.
The various scope sizes have their ups and downs. Some 6" refractors give the most beautifully pin-point star images you could hope for, which would be great for imaging, but could cost 15x as much as a 10" reflector which gathers more light is therefore better for tracking down faint fuzzies.
I have studiously avoided catching the imaging bug to date. I am particularly interested in visual observation of faint galaxies. In my experience you really need 18 plus inches to see any major detail in most of the well-known galaxies. I have enjoyed the best views of my observing life in a 20 inch Obsession style dobsonian reflector. It totally blows the views of a 10" scope out of the water. Whilst I have avoided catching the imaging bug, I have caught a fatal case of aperture fever, and am awaiting delivery of a 20" SDM scope.
There can be a great deal of variation in the quality of mirrors, and collimation can make a big difference. One 18" scope that I know of has a very poor mirror, and for that reason I would compare the views from it to a good quality 10 - 12" mirror.
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