I have looked though the website and have not found any information of making the thumbnails bigger as the current size is just too small for me. I hope there is a simple solution to this. I'm thinking a good size would be 160x90 pixels.

Depending on your theme you would just change the thumbsize option. Be aware, though, that page layout of the themes is typically setup to expect a certain size thumb so you may have to alter the theme CSS if you change the size.

I can't seem to find the exact place to change the thumbsize....., I know I must be blind, but i don't se it. I am using zenpage.

Thanks !

Depends on which thumbnails, the album thumbnails or the image thumbnails. Please read the theming tutorial if you haven't already.