She is not too big if you are talking "standard" bantam...yes a mix in terms, but I mean bantam size bantam vs. something like a Seabright which is tiny. She could hatch tiny Seabright eggs too, but she might crush the tiny chicks just moving around.
So back to bantam...I only have Silkies currently laying, so I don't have a size comparison other than a Silkie, whose egg is about half the size of a large/extra large store bought type. Physically she could cover quite a few eggs. She could cover about a dozen normal eggs, so she could physically cover about 16 to 18 bantam eggs...but that is a LOT of babies to care for...I think I would do no more than 12.
As to mixing, yes you can. I've mixed hatches with game size eggs and large. But the babies were about 3/4 the size of the large, so it wasn't a big deal.
Again if you are talking little bantams, there could be more of a problem.
LofMc