Usually 18mm but there are some 20mm ones about.
Cutwel [ one L ] do them.
http://www.cutweltools.co.uk/home/milling-cutters/hss-8-cobalt-k2-coated-hsse-cutters
Most are 18mm but if you look at the multiflute, standard length section they list one at 20mm, these are the cheaper HSS ones.
Modern carbide cutters tend to have same shank as the cutter because the ER series collets are now the defacto standard and holding say a 7mm cutter with a 7 mm shank is no problem.