I'd expect this is very high power since these spec's warrant a low cost printed circuit patch array considering the 4% bandwidth. If it's not high power, then whoever said to make a slotted waveguide array instead of a patch array is wacky.
There are computer codes for designing this.
Ansoft HFSS is one of the best. We've used it on waveguide arrays.
You need to add sidelobe spec's and polarizations to set up the problem, plus a physical footprint.
kch