My antenna theory knowledge is a little weak so I thought you guys
might be able to help.

Background:
I have a shed about 50 feet away from my house that contains water
pumps and similar equipment that I monitor. The shed has no windows
but was built using normal stud-wall construction. I built a small
device that receives information from sensors and then reports back to
my PC via my home network. I have no way to run wires to the shed
because there is a driveway in between, so I decided to use wi-fi. The
shed had a good strong wi-fi signal inside it and everything worked
well for the last 5 years. Recently, vinyl siding was added to the
shed. The installers used foil coated styrofoam insulation under the
siding. The shed has changed from being fairly transparent to RF to
being a somewhat leaky faraday shield. The wifi module still works,
but just barely. There are lots of missed messages and any
interference from devices like the microwave oven in the house shut
down communications.

Question:
I want to create an RF "window" in the shed wall. How big does it
need to be to let 2400 MHz signals pass through it? A full
wavelength? Half wavelength? Should it be rectangular or would a
vertical slot work? Opening the shed door (a full size metal door)
temporarily restores a strong signal, but I was hoping a much smaller
opening would work equally well. For aesthetic reasons, I can't
experiment by cutting various size holes. What's the smallest hole I
can cut in the foil coated insulation and be reasonably assured it
will work?

Thanks,
Pat