It works fine for me with Trac 1.2-stable.
diagram_size = 3,3 -> 288 x 218
diagram_size = 6,6 -> 576 x 436
diagram_size = 12, 12 -> 945 x 715
diagram_size = 24, 24 -> 945 x 715
and with each change the image approximately doubles in size, up to a limit.
Try deleting the png's in htdocs/tracworkflowadmin.
Tested with Firefox 52. Please try with Firefox or Chrome and make sure you are running the latest version of the plugin.