Boosting wifi antenna with aluminum foil: ideal placement/size?
I appears you are making a simple reflector.  In that case, the optimum shape is a parabola with the antenna at the focus and the opening pointing to the other antenna you wish to communicate with.

The wavelength of 2.4 GHz is about 5 inches, so it would be a good idea to keep the reflector at least that far away from the antenna.  Even then, you will get some diffraction effects.

Keep in mind that what you are doing may be OK for one-off experimenting but is not legal.  The laptop received regulatory approval (FCC if in the US) as built, not with a reflector installed.  A good reflector will cause a signficantly higher field strength in a particular direction, which most likely violates the maximum allowed field strength for the band, especially if the WiFi access point is distant and the laptop cranks up the power to the limit to try to get thru.