I'd definitely call it an Amaryllis belladonna as well. I have
belladonnas that range from 12-18 inches in height to around 3 foot or
more..... flowers range dramatically in size and colouration as well.
Also depending on whether involving multiflora breeding they can have
a few flowers all facing the same way, or 25 or more flowers facing in
a full circle. So there is a LOT of variability in belladonnas!!
<grin>

Your pic looks a lot like my "miniature" which has smaller flowers
than the "normal" belldonna and much smaller leaves. Looking at the
leaves they're so much smaller than the belladonnas, but the flowers
are much closer to normal size. If my form is around then I am sure
that there would be others with smaller leaves and quite probably
smaller flowers.

Hopefully this is of some help?