Martinetti,
I'm sure no great birder, but some of the best fun I ever had was last summer, watching a pair of Great Horneds raise their brood. We'd start a bit after sunset and go as late as we could see. We were about 250 yds away and I hope we didn't annoy them too much. They seemed to get used to us. But, this year they are nesting someplace else, nearby because I see them occasionally, but I don't know where.
Anyhow, what size exit pupil works best depends on your eyes and how dark you mean by low light. And then, however much magnification you are comfortable with. If your eyes open up enough to let in the 8x42's entire 5.25mm exit pupil, it will give a 72% brighter image than an 8x32, and that ain't hay. I have seen studies of iris size in dying light, and I think an 8x42 would be enough for most people. I was satisfied with mine, although it isn't a particularly bright one. I have not used an 8x32. "Twilight factor" keeps climbing with magnification and aperture of course, but I find a 10x50 isn't worth the trouble.
One thing that was very important, at least in the setting where I was, was control of stray light from outside the field of view. The owls were often in dark holes in a cliff, against the still fairly bright western sky. That is a hard thing to simulate in a store. In the parking lot at dusk maybe. But mail ordering and trying the bino out in your own way would be ideal.
Ron