NextGen Galley Resizing

Hi,
Please forgive my direct approach requesting help.
I am new to WordPress and have tried to solve my problem.
I have installed Net Gen free image gallery.
I want to resize the picture and buttons to make them smaller on the page so it fits in the middle of the screen on all devices.
I don’t want to scroll down for the navigation buttons.
When i resize I can’t get it to stay in the middle.
I am sure very simple for someone who knows what they are doing but I have been trying with my limited CSS knowlodge.
I don’t like asking for free help but if anybody knows a quick solution I would very much appreciate it.
Thank you

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