with 3.3m, it is still close to a Mini or a Fiat 500. There is not really a limit defined, but I think that such small cars are better as "supermini" than "hatchback" (same for the Twingo, so I would say that the limit is around 3.5m)
In Japan, there is a limit for these, for tax purposes. However, it evolved since the origins. In the 80's, kei-cars had to be under 3,20 metre long, now it's under 3,40...