I can't understand why an integral of a fraction doesn't resize the integral symbol by itself.
$\text{SAR}=\int_{campione}\dfrac{\bf{\sigma}(r)|\bf{E}(r)|^2}{\bf{\rho}(r)}dr$
I saw this Big integral sign but with bigints nothing changes (and yes, I loaded the package and I erased all those files that work like a cache). I wouldn't write an instruction to make this resize
You simply need
$\text{SAR}=\displaystyle\int_{\mathit{campione}}\dfrac{\bf{\sigma}(r)|\bf{E}(r)|^2}{\bf{\rho}(r)}\,dr$,
if it really is an inline, not displayed, math. If not, a solution may be as follows:
\[
\text{SAR}=\int_{\mathit{campione}}\dfrac{\bf{\sigma}(r)|\bf{E}(r)|^2}{\bf{\rho}(r)}\,dr
\]
Using \bf in LaTeX is not recommended. You have, e.g., \mathbf{}.
\dfracin an inline math expression is almost surely an error. If you need to use\dfrac, switch to displaymath style; a side benefit will be that the integral symbol becomes large automatically. – Mico Jun 4 '16 at 17:57\bf{\sigma}won't produce a bold σ. For this you should use\boldsymbol{\sigma}or\bm{\sigma}if you load thebmpackage. – Bernard Jun 4 '16 at 18:14\bfinstead\bm– Shika93 Jun 4 '16 at 18:20