According to Orange literature of the day the clockwork came in 15, 17, 19 and 21.
The Elite (O, prestige) came in 17, 19 and 21.
The geometry, apart from the early clockwork frames (flat top tube) was near the same for them all. The 19's have a 120mm headtube with a deifnitive gap between top and downtube as they abut. The 19's have an almost flat top tube the 17's a kona-esque slope (and the top and downtube almost touch) and the 15's almost a straight line top tube with the seat stay.
I've got an Elite in 19, Hawki and BoyOboy have a 17 (i.e. the only one smaller) and there's a few 21" on here.
As far as I know all but a few of the earliest frames (formula?) are measured c to top.
but others on here know far more than me so I'll stand corrected.
And edited re earlier frames
Like a fish ever needed a bicycle anyway...MTB: '89 MF Alu Team Pro,'88 Muddy Fox Courier,, '92 Orange Elite (now 17"!), '04 Kona Fire Mountain (now Son's), '96 Muddy Fox Ranger home-build (son's), Specialized RockHopper '92 (Ad's retro-build) ,'02 Kona Scrap Special Edition OB (3rd Son's), 1989 Orange Clockwork (Gone!?)
ROAD 2013 Orange CarbO ,2012 Basso Devil (3rd son's), Colnago CLX3.0 Di2 self-build