Size of Formulas with no negative sign for Matrix Permanent
Snir has proved a tight lower bound on the size of monotone formulas representing the permanent of an $n\times n$ matrix. The lower bound is $2^{2n - 0.25\log^2 n}$, and he notes that a formula of size $2^{2n - 0.25\log^2 n + O(\log n)}$ exists (Theorem 3.1. and comment after the proof). 

The survey by Shpilka, and Yehudayoff is a good resource. Also, a lower bound of $2^{\Omega(n)}$ on the size of monotone circuits for the permanent is known as well (proved by Jerrum and Snir)