.. make up some numbers for a widebody and see what you get .. at least that will give you a bit of a feel for the problem you pose ... for the sake of discussion, let's run with the 744 ..
(a) check out a website which will give you an idea of the dimensions .. a quick search on Google gave me
this one from which I scaled off some guess numbers to use in the calculation below ... for the sake of the discussion, let's presume that the CG and datum are co-located somewhere near the inboard engines ... so we might have an aft-located seat somewhere near 25 metres aft of the selected datum.
(b) similarly for weight we could use the information in the previous url to pick, say, 350,000 kg
Simple now to run a calculation for the addition of an 80kg passenger somewhere down in the back of steerage ..
350000*****0*****0
***80*****25**2000
350080**0.006**2000
(.. one of these days I'll find out how to format numbers in columns in PPRuNe .. for the moment please excuse my lack of finesse ..)
For an aircraft of this size, a calculated CG change in the vicinity of 5mm is going to be lost in the normal error clutter so a reasonable answer to the question is it doesn't affect things to any significant extent for a widebody ....