Designing castellations/half holes using EAGLE
If via ring is small and there is enough space, there is no limit for the ring size. But if there is not enough space coupled with dense lines, the ring size should be over 0.1mm.Min. 
Size of drill via CNC is 0.3mm. 6.3mm is maximum size.
Half-hole is a special technology and the min. aperture must be over 0.6 mm
it is our pcb board capabilities. you can see it as reference.