The length of the arm is set by a number after the angle of the arm: :angle,length. I used 0.6 for you.
The style of the dots can be changed with the fourth argument of \setlewis.
I didn't find a "pretty" way of changing the font color, so I just used \textcolor :P
The font size can be changed by changing the \printatom macro, inserting the font size before the \ensuremath.

\documentclass{article}
\usepackage{chemfig}
\usepackage{bm}
\setlewis[]{}{}{fill=yellow,very thin}
%\renewcommand*\printatom[1]{\large\ensuremath{\mathrm{#1}}}
\renewcommand*\printatom[1]{\large\ensuremath{\bm{\mathsf{#1}}}}
\begin{document}
\centering
\scalebox{6.5}{%
\chemfig{%
\textcolor{green}{B}
( -[: 90,0.6]\lewis{0:2:4:,\textcolor{orange}{F}})
( -[: -40,0.65]\lewis{0:4:6:,\textcolor{orange}{F}})
( -[:-140,0.65]\lewis{0:4:6:,\textcolor{orange}{F}})
}%
}
\end{document}