Most commonly available magnets are rectangular though...
Far as I can tell given the rectangular 2:1 ratio limitation, the inside corners should touch each other, or perhaps even shave the corners off to push them closer together, depending on the pole count. if the pole count is nearly infinite, they should be spaced about 1/8th of their width, apart. this fits the standard 150/180 degree arc that nearly every electrical machine uses. while the exact arc is selected to minimize cogging for iron cores, anything significantly less reduces the voltage.
However, given copper is one third the price per cubic inch of neodymium magnet.. there may be some marginal gain to be made by increasing the diameter. but the only reason i can come up with is because it increases the surface area available for cooling the coil.*
I have said this years ago, but the coils should be overlapping, trapezoidal and pressed. thanks to three-d printers you could print coil forms to wind the coils on, but you can make them from wood too. when you have an overlapping coil set, you get nearly double the copper density compared to discrete single coils spaced out like donuts on a tray.
with the large inside to outside diameters made by using 2 or 3 inch long magnets and >12 inch diameter disks, you have no shortage of space on the ID for the end turns of the copper coils, unlike the iron core examples where the optimal figures are more like 12 inch diameter disk, and 4 inch long pie shaped magnets.
*and i'm not sure this is helpful either because reduced emf and higher harmonic content only makes matters worse.
on the plus side however, more copper equals more thermal mass. so the greater thermal mass may offset reduced efficiency.