As pointed out by verktyg, there were several other sizes of tubular tyres, When I was racing in the early 1970s there was a Pee-Wee class racer (10-11 years old) with a custom built Reynolds 531 DB frame with full Campagnolo (Nuovo) Record group (yes, even the brakes) and 24" tubulars. His bicycle was the envy of a lot of riders.