Circuit size class ($\mathrm{SIZE}(T(n))$)

The languages whose length-$n$ indicator functions have Boolean circuits with at most $T(n)$ gates for sufficiently large $n$, over a fixed bounded-fan-in complete basis. The family need not be constructible by one algorithm. Polynomial-size bounds give P/poly; the truth-table upper bound for circuit size applies even to undecidable languages.

OR together one minterm for each accepted length-$n$ input. Each minterm is an AND of the appropriate Boolean literals. There are at most $2^n$ minterms, and their negated input wires can be shared, giving $O(n{2}^n)$ bounded-fan-in gates. This is a nonuniform existence statement, not an algorithm for determining an undecidable truth table.