Probably the "mag" multiplier. It should be around 50.000, I guess. I'll have a closer look tomorrow.
That was is; it's fixed on the wiki. Olivier can enjoy his line of little skulls with the correct metrics ... By the way, I thought the test on size was wrong too [if (size < 0) then size = (- 655.36) * size end] because, from reading the LuaTeX reference, I assumed that a size of -1 meant an actual size of design_size, thus defaulting to 10pt == 655360sp. But after testing, it seems that it's actually not the case and that that number has to be divided by 1000. Am I mystified or should the paragraph at the end of page 50 of the manual be corrected? I do remember having read something about a factor 1000 in the past ... (like Metafont's mag parameter). Arthur