For a bounded subset of a metric space the diameter is two times the radius!
Let $S\subset X$ be bounded. The definitions are:
$\mathrm{diameter}(S):=\sup\{d(x,y)\,|\,x,y\in S\}$
$\mathrm{radius}(S):=\inf\{r>0\,|\,\exists x\in X:\,S\subset B(x,r)\}$
where $B(x,r)$ denotes the open ball of radius $r$ around $x$.
