I don't really know how the increase is calculated, but I would say it should be 1.5x or even 2x (maybe too much; 2x) the current daily speed increase, because it's not so fun to have so many people around the plague, and considering the -understandable- limits put to growth beyond plague, something must be done on this matter. This way growth would be neither "boring" (spend on UP/Covert due to very limiting plague growth) nor "only for the richest" (like Phoenix's suggestion seems to be to me, about increasing the limits by 5x at once).
Also, while doing that, increase at the same speed and % the trade/raid limits. But not so sure about it.
note: if you total population size is greater than 575,890,411 disease will surely strike your lands. Above 431,917,808 and raiding is no longer possible. Above 287,945,205 and trading/buying units no longer possible.