The problem can be modeled as a linear program. For each of the $n$ posters to be added to the wall, we introduce the variable $A_i$ denoting the magnification of poster $i$. The sum of perimeters of all new posters is
$$2 \sum_{i = 1}^n A_i (w + h)$$
which we maximize.
As constraints we need to ensure that the new posters do not overlap with each other and the existing posters.
We need to make sure that one of the following holds:
$$2 |x_i - x_j| \geq A_i w + w \ 2 |y_i - y_j| \geq A_i h + h.$$
This is equivalent to:
$$A_i \leq \max {2 \frac{|x_i - x_j|}{w} - 1, 2 \frac{|y_i - y_j|}{h} - 1}.$$
For the new posters we obtain
$$A_i + A_j \leq \max {2 \frac{|x_i - x_j|}{w}, 2 \frac{|y_i - y_j|}{h}}$$
analogously.