250g = 2-3 pieces (approx.)
Banana Shallots are longer than the ordinary round shallots. They're supposedly named for their shape, though they're not really banana shaped as in with a curve and all; they're more torpedo-shaped -- though presumably someone decided the term "torpedo shallots" would be off-putting.
They have smooth, tan-coloured skin. Some varieties have almost all white flesh inside, others are white at the core with the outer layers of flesh being pink, purple, etc.