Number of minimal DFAs of size at most $m$?
According to Ishigami Y., Tani S. (1993) The VC-dimension of finite automata with $n$ states,
http://link.springer.com/chapter/10.1007/3-540-57370-4_58
,
the VC-dimension of the concept class of $n$-state DFAs over an alphabet of size $k$ is
$$ d=d(n,k) := (k-1+o(1))n\log_2 n.$$
It follows that there are at least $2^d$ distinct $n$-state automata on a $k$-letter alphabet. The upper bound on the number such automata follows from a simple counting argument (given in the paper), and is at most $2^d$.