You might consider using instead of chkboxes/optgroups only buttons and radiobuttons for mobile GUI input.
IMO they are more appropriate for fumble fingers and more self-contained ;-)
As the current motto is "mobile first", do such a version and check if the GUI is so bad on desktop that you will support slightly different GUIs.
RO GUI controls with longer texts might stil be better served by graph styled chkboxes, although I guess that radiobuttons are still better than optgroups.

You might also implement a check in your drop down lstboxes and cbo that the amount of items is small enough to fit on screen in mobile: At least I find scrolling there more unpleasant.

my 0.02€

thomas

>What property defines a checkbox control size? I mean the image of the check box itself. When I increase the FontFize it only affects the caption. TIA