Mac Pro (Late 2013) with new system installed can't get past first page

I bought a Mac Pro 6.1 and it was delivered to me with a newly installed system not yet registered. On boot-up it displays a very small image of the start-up page with the language options etc..BUT it's so small I can't see the (Apple)wired mouse pointer; The wired keyboard (Apple)doesn't register when I press return or cmd Q or escape. the up and down arrows are having no effect either.

I've tried booting from an external SSD that has 10.11.6 installed. I'm going out of the HDMI port to my monitor, an LG TV.

My Cheese-grater Mac Pro 2010 is also connected to the TV on an different HDMI port. It works flawlessly.

How can I change the screen resolution on the newly istalled (but not yet registered) system please ?

Basil