A knot in S^3 is small if its complement does not contain a closed incompressible surface. Is it a generic property for knots, meaning that among all knots with less than $n$ crossings, the proportion of small knots goes to 1 when $n$ goes to infinity?

In general, a knot is not small. Abigail Thompson showed that if a thin position is not a bridge position, then there exists a closed incompressible, nonperipheral, surface in the knot complement (Corollary 3). Thin position and bridge number for knots in the 3sphere We need to show that in general, a knot in a thin position is not in a bridge position. Even if it wasn't, FinkelsteinMoriah showed that there exists an essential meridional planar surface in the complement of a knot in a bridge position. Hence there exists a closed incompressible, nonperipheral, surface in the knot complement. 


Following up on the first comment by Misha: Your question is very sensitive to the model you choose. I vaguely remember a talk by Ken Millet (http://math.ucsb.edu/~millett/) in which he gave a natural model of knot generation where the generic knot seemed to be a connect sum of $O(n)$ copies of the trefoil. If you tweaked the model, then the generic knot was the unknot. And to reply to another comment above: one could condition on the knot being hyperbolic. Then the model is more interesting to analyze. However such a model is "unusable in practice"  you can't actually generate knots this way because the waiting time is too long. 

