Difference between $\mathsf{SIZE}(n^k)$ vs $\mathsf{P/poly}$ and $\mathsf{SIZE}(n)$ vs linear size circuit?
$\mathsf{P/Poly} = \bigcup\limits_{k\in\mathbb{N}}\mathsf{SIZE}(n^k)$. 

We don't know if every language in $\Sigma_2$ has a polynomial size circuit, but we do know that we cannot have polynomial circuits of bounded degree,i.e. $\mathsf{SIZE}(n^k)$ for some $k\in \mathbb{N}$, for all languages in $\Sigma_2$ (Kannan's theorem).