P trap installation trouble-HELP!

Hi
Not much of a DIYer, but here goes:
trying to install new sink in upstairs bathroom. Old one had a semi-flexible rubber P trap. When I removed it I thought, no problem. I can replace this with chrome.
Not so. The stub-out(?) from the wall is roughly 1 1/2 to 2 inches long, and is 1 1/4 copper. It appears as though when installing the previous sink, the people just cut the pipe with a hacksaw as there are no threads on the pipe. A 1 1/4 compression seal is too small, a 1 1/2 is too large.
Do I have to solder on a new (coupling? I think is what it's called)? The wall is faux beadboard over plaster, and due to the proximity I am worried about blackening or even setting fire to the wall.
My wife is losing patience with me, and I am almost at the "Let's just phone a plumber" point.

Measure the OD of the copper stub out...
If is it 1 3/8", you have 1 1/4 copper plumbing "pipe" to tie into...
There is a solder-on connector to convert to the tubing size for ptraps..
You might want to consider hiring a plumber to do this as 1 1/4" brass W&V connectors are a little hard to solder with the typical homeowners' torch...
If you do want to try it, make sure you don't sear the wall - use a heat shield.
As well, make sure you have a MAPP torch instead of a propane one (more heat).

drain

I am almost at the "Let's just phone a plumber" point.

you will probably save a lot of time, frustration, if not money if you succomb to that feeling and call a plumber.

If you DIY , have a fire extinguisher handy.