It looks like it’s adjustable and has just fell down. Often the conical washer and ring are in the bottom half. As for size, that loos like a domestic kitchen sink waste, so usually 1.5” and you’d need a longer extension trap piece, not just waste pipe. I would try what I have said in my 1st statement.
All you need to know is what it's serving, if it's a bathroom basin then it'll be 32mm, if it's a kitchen sink which that is or a bath etc then it'll be 40mm.
As suggested, I guess it had just come loose and you just took it apart and remade?
The question about - are you a plumber - is quite valid if you are asking questions like that, have you had any training? That kinda stuff can't get any more basic.
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