About 2 cm (when I found them). They are olive green. They have a black head, and black dots on the side, around the spiracles. They are not hairy. If I counted right, they have three pairs of prolegs (fake legs).
I found them in a spindle (euonymus), and it seems it's the only plant they like. The spindle is close (within 1 yd) to a sunflower, an acer japonica, a hawthorn and a couple of other plants, all of which are untouched.
The caterpillars are in groups of about 20 nesting and eating at the ends of the fresh branches. They have spun a spiderweb-like nest around them, although it seems they are not pupal yet, and they are still eating away. I've found about 4 of these 'communities' in what is yet just a small bush (about 2' high).
I have got quite a lot of sparrows, tits and other birds in my garden, but so far they haven't seemed to take a liking to these larvae. I'm in the Netherlands (zone 7).