Keith,
Please see attached image. W8.1 CSM3.9\4 most recent beta with custom image. Both default and DF taskbars double height. Note the position of the Start Button on DF taskbar in comparison to default in that it seem to align with top edge of taskbar rather than center as it does on the default.
Pete

More on this - if I choose the default image see difference everything is fine. It just seems to be with custom images. Tested a couple of other custom images they do the same. They work fine on Default TB;i.e. in the center but on top edge in DF TB.
Pete