Sizing Thumbnails Issue

I need help making the thumbnails much larger. I’ve been trying to select “large thumbnail” in the drop down menu, however nothing changes. I’ve tried deleting the media for a particular post and reloading it, but that doesn’t work.
I’m in the beginning stages of creating the website and just using stock photos in place for now.

I’ve been searching through all the topics posted but can’t seem to find any past solutions to help my problem. FYI I’m looking for the easiest solution as I am not very experienced at all!:)

Thanks for any advice.

The page I need help with: [log in to see the link]