I'm contemplating enlarging a small letterbox in a wooden door. I'm
not 100% sure that the site of the intended larger hole is completely
solid, which might make the proecess more difficult than otherwise, but
I suppose that all I can do is proceed carefully and - if necessary -
try to fill in gaps in the frame if I find some?

What's the "standard" way to enlarge an existing hole? Do I use a saw,
or drill lots of holes and then knock sections out with a chisel, or
what? Or a power jigsaw (carefully)? I'm not planning on taking the
door off its hinges unless that's absolutely necessary.

--
Jeremy C B Nicoll - my opinions are my own.