Image Re-sizer Problem

I have just worked out how to put a thumbnail image next to the link to a post by using the Custom field image.

However I am not sure how to re-size the image so that all the thumbnails for all my posts are the same size. I only want a small thumbnail about 150 x 150 but when you click on the post, the same image appears much lager.

I would also like them to be in a neat vertical line. An example of what I am trying to do is on https://www.paragamer.com

Please can someone tell me what I need to do.

Many thanks.