Usually it's the other way around: how to get other blackboard bold fonts instead of those provided by kpfonts. ;-)
Here's a way: find out what font family contains the characters, extract the information from the .fd file and add the scaling feature. Don't worry, I did it for you. ;-) Just decide what magnification factor you want instead of 1.3.
\documentclass{article}
\DeclareFontFamily{U}{jkpsyb}{}
\DeclareFontShape{U}{jkpsyb}{m}{n}{<-> s*[1.3] jkpsyb}{}
\DeclareFontShape{U}{jkpsyb}{bx}{n}{<-> s*[1.3] jkpbsyb}{}
\DeclareMathAlphabet{\mathbb}{U}{jkpsyb}{m}{n}
\SetMathAlphabet{\mathbb}{bold}{U}{jkpsyb}{bx}{n}
\begin{document}
$X\mathbb{R}Y$
\boldmath $X\mathbb{R}Y$
\end{document}

\DeclareMathSizes {t-size} {mt-size} {s-size} {ss-size}as explained here? Or do you want to scale only a few symbols? – Biswajit Banerjee May 21 '16 at 06:18