I just emptied mine - there were 3 large towels, 3 small towels and about a dozen pairs of underpants. This just sufficed to line the drum once the cycle had finished, so I could easily have fit three times that much in.
However, this was light-coloured stuff. I don't have that much light coloured things (light blue towels, only used when we have visitors, skin-coloured underpants)
I like to wash whites together, and wouldn't put light blues, beiges, etc, in with the dark coloured towels (we even have black towels! And some black duvet covers!) - so it's worth remembering this when buying new linens: try and keep to colours that will wash together to make for more economical washes.
Also - it's best to follow the instructions on the packet re quantities of washing powder - you obviously need more for 6-7 kg of towels than you do for 3 kg of polyester or mixed fibre blouses, but it's hard to judge how much more.