You will imagine us as bats, or something like. Flittering at dusk, with strange little faces. But it would be best if you think of birds, the tiny brown ones, virtually invisible, that flick at the edges of your vision as you pass a clump of willow or a coppiced ash. Think of a wren in a holly bush, a warbler scuttling down a thick trunk; think, and then, if you can, unimagine the shapes, and remember the movement.
Size varies, as in life, but I have never known anyone bigger than a gull, and she was a rarity, she didn’t stay long.