You can do an

$ apt-get source navit

then you should get the source and the build directory.

Then debian/rules contains the configuration

# scale all gui icons
DEB_CONFIGURE_EXTRA_FLAGS += --enable-svg2png-scaling="16 24 32 48 64" \
--enable-svg2png-scaling-flag="16 24 32" \
--enable-svg2png-scaling-nav="16 32 48 64"

I never changed that. Can you explain what the three parameters are doing and what settings you are recommending?

Thanks.