There is no largest area, but the area can be made as close as wanted to 75.

Let's imagine our rectangle. It has diagonal $AC$, centre $O$; let's say $AB$ is the longest side and $H$ is the middle of $AB$.

The rectangle will be folded along a line $p$, which is perpendicular to $AC$ at point $O$. $p$ also crosses $AB$ at some point $M$.

It is easy to note that when we fold the overlapped area will be of size $2*Size(AOM)$. The pentagon area is $Sp = 100-2*Size(AOM)$. In order to achieve biggest pentagon we need to make overlap as small as possible.

But, $AOM$ always includes $AOH$ and $Size(AOH) = \frac{100}{8}$. Thereby $Sp = 100-2*Size(AOM) < 100-\frac{100}{4} = 75$.

Finally, let's note that making $\frac{AB}{AD}$ ratio big enough we can make point $M$ as close as needed to point $H$ making $Size(AOM)$ as close as needed to $\frac{100}{8}$ and thereby $Sp$ to 75.