Unfortunately based up north, but have an XS500.
Would say internal space depends upon chosen seating layout, although at 2.2m beam and large tubes the internal width is 1.2m. The 515 range is slightly wider at 1.25 and the 550 with its 2.6m beam has a 1.5m wide deck.
We went for a single jockey on our build to maximise open deck space, with the option of fitting a second jockey behind at a later date. More than often running solo or occasionally max three aboard.
Never fitted the extra jockey, and now carry a large storage box aboard. Passengers can sit on tubes or inboard on box, both with good handholds on jockey backrest. Not everyone’s taste but works for us.