A graph definition allows to define a width and and height. This property is ignored in the WebUI.
It was possible to overwrite the whole RRD command with something like this:
report.mib2.HCbits.command=--title="Bits In/Out (High Speed)" \
--vertical-label="Bits per second" \
--width 700 \
--height 300 \
This forces RRD "always" to generate the given graph size. Specifying the size in the prefab graph definition allows more flexibility to decide in the view if the default size or the graph specific width and height should be used.