The Airbus ACAP document shows standard A380 economy-class seat widths as 18", with an overall width across the seat row (armrest to armrest) of 248".
I'm aware, though, that airlines like SQ and AF are putting 19" seats into their aircraft. e.g.
SQ explicitly say that the 19" width is between the armrests
That's a lot of extra seat-width over the ACAP illustration
I'm not sure what aisle width SQ have, but 10-abreast at 19" seat bottom width would seem to indicate that 17" - 17.2" is possible on the main-deck at 11-abreast, with basically the same aisle widths.
I'd welcome some informed commentary