I am using Win10 (64bit) PC and have imported 4k video from my Sony 4k camera

No issues with editing, etc but when I export it (tried several 4k formats), I have black bars all around the video content, yet the titles & transitions do take up the full height of the screen - see below.

The screenshots above were captured through the same window in the same position.

Any ideas as to how to increase the size of the video content ?

What are you viewing the finished exported video with? I would bet VLC.

Try WMP.

Thanks Tony

I was actually using WMP.   I have re-exported the video again and this seemed to have fixed it. Not sure why as I used the same settings