If you use a humbucker mounting ring you can be generous with regard to the size of the pickup cavity. Obvious limitations are that the ring must cover the whole cavity and that there is still enough wood left in the corners for screwing down the mounting ring.
The cavity for a humbucking pickup (grey area) is shaped according to the mounting ring (white area) used. The "ears" can be routed deeper if necessary.
It is advisable to pre-drill pickup cavities before cutting them out. Set the stop to the point of the bit. Then cut out the cavity using a template and a flush-trimming cutter with shank-mounted ball bearing. The few millimeters still left due to the point of the drill bit can be routed off at the end. Small-radius corners are best pre-drilled using a bit that has the right radius.
Excerpt from my book Building Electric Guitars
Order a "Pattern Flush Trim Router Bit" at rockler.com