If a standard Tcl expression does not allow you to calculate the symbol size as you wish then you may be able to invoke your own Tcl procedure to perform the calculation. This facility is only available if you are using SkyCat as a client and is only likely to be useful with your own catalogues. An example might be:
symbol "rmag bmag" circle "[my_plot_proc $rmag $bmag]"
where you provide Tcl procedure my_plot_proc.
Writing Catalogue and Image Servers for GAIA and CURSA