Ok, i did some research, and by research, i mean i booted up my DOA 3, DOA 2U and DOA 4 copies, and while i will show the footage here once i edit everything together. I can safely say without any doubt. DOA 5 and DOA 6 stages aren't small compared to previous titles, not much at least.
DOA 2 U, arenas are actually very small, with some biggers, there is barely any kind of slope or uneven terrain in the stages, there is a lot of transitions though, some more than others.
DOA 3 has some big areas like Lorelei, but that is in DOA 5 as well exactly with the same size and areas. Zack island in doa 3 is considerably smaller than DOA5's version as well. It also features a lot of just small arenas as stages.
DOA 4... Well, not that big either, seaside market, the upper area is kind of small, and the lower area is just doa 3 zack island. The big stage in that game is certainly that tokyo stage, but the bigger area is not that big again, and there is two transitions to two super small areas. The street stage, i laughed, it is super small, with invisible walls on both sides, and random cars appearing from all sides, whoever thought that was a good idea needs to get his concepts right.
I haven't played DOA 6 yet, but i've seen enough footage, and the stages, and i mean that collision, space/wise, are DOA 2U/DOA 3 stages through and through (with the DOA 5 danger zones added in in some places). The dinosaur stage has the first area with few slopes, water terrain, and is the size of a regular arena in DOA 2 U, the transition goes into a bigger, giant arena that is way bigger than the octagon stage in DOA 3 for instance. Miyabi, while we haven't seen the bulk of it, seems comparable in size to the DOA 4 upper area of the tokyo stage, just without the fences, and then it transitions into Hidden garden, which is a small arena again.
Say what you want about how those stages look, art direction and usage of colors and lighting, that stuff matters and you're on your right if DOA 6 doesn't appeal to you visually, but when it comes to collision, space, gameplay, DOA 6 stages are just as big, small and varied as DOA 2 U and DOA 3. And while some people like to praise DOA 4 stages, the stages that it does different from doa 3 and doa 2 u, are actually terrible stages. And we have to keep in mind that there are stages that we haven't seen yet.
EDIT:
Here the video i promised: