Wondering if it is best to build a new dashboard on the mobile device it will be used on or is there an easier way to build it on my PC then use it on mobile?
If on the PC what do I need to do to make sure everything fits on the mobile device?
I would still suggest building the dashboard on a PC, navigating the interface is easier with a keyboard and mouse on a big screen. When setting it up, you can set the tile width to zero and tiles will auto-fit to the size of the display. I would suggest building dashboards that are tailored to your mobile, but that can be used on a PC / tablet if you wish.
Also check out [RELEASE] Hubitat Dashboard - Android dashboard app as an alternative playground. I haven't tried it yet myself, but it's always good to have options if you wind up committed to doing things on the mobile side.
And if you're looking for tried-and-true suggestions from slightly off the beaten path, with vastly steeper learning curves, these come highly recommended: Home Remote and Homer.