Let F be a field of characteristic $p > 0$. Let $g$ be a linear lie algebra, that is there exists a natural number $n$ such that $g \subset M_n(F)$. Does there exist a condition involving $n$ and $p$ such that $g$ is semisimple if and only if its killing form is nondegenerate?
The short answer is no. In prime characteristic, the Killing form sometimes behaves badly even for simple Lie algebras. If "semisimple" means that the solvable radical is zero, there is no way to obtain the classical equivalences with nondegeneracy of the Killing form and with the direct sum decomposition into simples. Moreover, the simple Lie algebras have only recently been classified when $p=5$ (PremetStrade), while for $p=2,3$ little is known and for $p>5$ the classification takes an enormous amount of work (by BlockWilson and others). Conditions on the dimension of a faithful representation or on the prime are not enough to sort out the concept of semisimplicity. Still, a lot is known. For example, Seligman and others explored in the 1960s the class of modular Lie algebras for which the Killing form is nondegenerate. ADDED: Much more could be said along these lines, but for older results see the book Modular Lie Algebras by G.B. Seligman (Springer, 1967). Modular Lie algebras have been of much less importance overall than linear algebraic groups, whose Lie algebras are "restricted" (have a nice $p$th power operation) but still don't reflect precisely the group structure or representation theory. Moreover, the representations of simple or more generally semisimple Lie algebras in prime characteristic are poorly understood even for those arising from Lie algebras of semisimple algebraic groups. So working with a fixed $p$ and fixed $n$ will usually not be illuminating. 


This started out as a comment on Jim's answer, but it got too long (EDIT : It looks like Jim edited his answer to say some of this while I wrote this up). First, I recommend Seligman's book "Modular Lie Algebras" for a nice account of what happens to Lie algebras in positive char if you assume that the Killing form is nondegenerate. It's maybe also worth giving an example to show what can happen (there might be easier ones, but this came up in a paper I wrote a while ago and gave me no end of headaches). Consider $\mathfrak{sp}_{2g}(\mathbb{Z}/p\mathbb{Z})$. It is not hard to show that this is simple for $p > 2$. For instance, an argument is contained in the paper N. Jacobson, Classes of restricted Lie algebras of characteristic p. I, Amer. J. Math. 63 (1941), 481–515. By the way, this is false for $p=2$. Anyway, one can calculate the Killing form, and it turns out to be degenerate exactly when $p$ divides $g+1$. One interesting observation, however, is that we are really looking at the wrong bilinear form. Elements of $\mathfrak{sp}_{2g}(\mathbb{Z}/p\mathbb{Z})$ are matrices, so we can define a blinear form $$(A,B) = \text{Tr}(AB).$$ This is NOT the Killing form, as we are not looking at the adjoint representation. One can then show that this bilinear form is nondegenerate for all $p>2$. 

