Number of states in DFA equivalent to given NFA
The standard powerset construction takes an NFA with $n$ states over an arbitrary alphabet and produces a DFA with $2^n$ states; this is tight for alphabets containing at least two symbols (see Chrobak's paper below for several references for this).

As for unary alphabets, Chrobak showed in his paper Finite automata and unary languages that a unary NFA with $n$ states can be converted to a unary DFA with $O(e^{\sqrt{n\log n}})$ states, and showed that this is tight (up to the hidden constant factor).