I still believe that here feet would have been small, so a 'child's nine' (as earlier). This is not only in line with the description:
Light she was and like a fairy
But if she had great big clod-hoppers, she would be unlikely to be tripped up by just a splinter:
Hit her foot against a splinter
Fell into the foaming brine.

UK women's size 9 = EU size 42-43
UK Child's size 9 = EU size 27