If I were you I'd get him back to do the job properly, I wouldn't hold out too much hope though - if he was such a muppett that he couldn't recognise the fact that he cocked it up the first time, he probably won't see why you want it sorted now. Good luck anyway.
Hmmm! what was the old size? he probably went with that? but on saying that it should really be slightly proud of the plaster. You can ask him to rectify and if he's an honest joe? he'll redo it although that would mean him buying new wood and his time. :-)
Bit weird that he didn't fit the architrave while he was there....
Anyhoot, I faced the exact same problem just t'other day. Had a shed load of work done recently (new 2 storey extension) and as I work all the hours God sends to pay for it, I couldn't be on the back of the various trades when it came to snagging.
Just started decorating downstairs loo and discovered that the doorframe had been fitted on the piss, such that at one point the plaster was about 10mm proud. The chippy had tacked the architrave in place, but for some reason (time, probably) had not addressed the plaster issue.
I removed the architrave and hacked off the plaster so that I could properly fit the architrave. I then replastered with OneCoat (or somesuch - a Hanson product that seems to do what it says on the packet) and so, in effect, the architrave is "buried" in the plaster. But it looks fine. If you have any skill with a filling knife, you could do the same, and no-one would be any the wiser.
If you are looking for a DIY solution as opposed to getting the joiner back to do it properly then I has a similar situation a year or so ago. Not due to bad workmanship but because I had had a room re-skimmed and the new plaster was proud of the old door-frame. I ripped some thin strips from some old floorboards and glued and tacked them to the edge of the frame. Brought it up flush with the plaster so that the new architrave would fit. Does help to have a table saw though but it could be done by hand.
A quick fix might be to get strips of wood of the right - or slightly larger thickness. B&Q etc sell them in the mouldings section. Pin and glue them flush to the 'door' edge of the frame and pin the architrave to that. More expensive than making your own strips to the exact size but easier if you don't have the required tools. If the architrave is now slightly proud of the plaster you can make good the gap with filler.
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