The question is a little vague. First class can mean a number of things from a deluxe sleeper bed/cabin on some intercontinental flights to seat that is not much more than an economy one on a short haul flight. Also, first class has changed over time, so those 66 seats on a 747 on AA
were probably not much more than a premium economy seat today. Concorde had 100 seats, but while they were not first class in dimension, the service was above and beyond.
In terms of proportion of the available cabin space, first class had its peak in the 1950's and 1960's when whole aircraft like Britannias or Constellations could be configured in a 100% first class configuration. The original configurations for the 707/727 often had over half the floor space devoted to first class.
One also has to look at the likes of Maxjet, EOS and the other all premium carriers flying the Atlantic. Who can forget MGM Grand Air either on the glamourous LAX
Maybe you could refine the question. Are you talking floor space, proportion of passengers or simply over numbers of passengers flying up front?