Oh, I also found this function:
Luxor.Cairo.get_layout_size((currentdrawing()).cr)
which looks like it returns some kind of Pango layout dimensions:
julia-1.10> setfont("", 30)
julia-1.10> w, h = Luxor.Cairo.get_layout_size(currentdrawing().cr)
2-element Vector{Int32}:
0
30