There are 3 ways to achieve this:
If you shutdown Dashboard in fullscreen-mode, it will start up again that way.
You can also save it as part of a layout and tell DashBoard to restore that layout on startup by adding -layout LAYOUTNAME to your shortcut (make sure you tell uncheck "Maintain current state" in the layouts view).
Finally, as of DashBoard 8.3.1, you can also add -fullscreen true to your DashBoard shortcut to launch DashBoard in fullscreen mode.
#DashBoard