The adsorption transition of knotted polymers and the length of the knots in the adsorbed phase is studied via MC simulations of a lattice model. The motivation comes from the different results obtained for 3D "real" knots and for the "flat knots" model: these differences make not clear which behaviour should be observed, and in fact we find a nontrivial result, namely knots in the adsorbed phase do not show immediately (i.e. for T<Tc) the flat knot behaviour, which is expected for T=0. Our results also show that the behaviour is independent on the specific prime knot embedded. Very recent experiments may show an agreements between our results.