The mat is oversized so you don't have to worry about precise placement every time. You can pretty much just toss it back on the plate and you'll be within the print area 99.99% of the time. Pretty smart design.
@mc_ott has figured out (and I can confirm) that the FLUX Delta is "capable" of printing a bit beyond the 170mm diameter, it is a firmware limit. Most likely to ensure hardware tolerances and keep everything uniform as some machines may be able to push it a bit more than others. We're only talking about 1cm, maybe 2cm if you are really lucky, but that machine would be rare. And even pushing it past 170 is going to potentially put some weird angle stresses that would cause premature wear. I don't forsee FLUX ever lifting that limitation, any gains wouldn't really be worth the downsides.
As far as 20x20x23, not going to happen without some serious mods, and not sure how you could ever gain 2cm at the top end without changing rods, belts, etc., but then why stop at 2cm?