I have html_meta_tags active, which is creating all the meta and og tags I need, however, I don't know how or why the og:image is using the size thumbnail that it is (90px or so).

Apologies if this is a simple question, but I can't seem to figure out how to set or change the dimensions in the admin, and after quite a bit of googling/searching the forums, I still haven't come up with a solution.

The plugin has the options "Open graph image - width/height" actually that should be used if og:image is enabled. Default if empty otherwise is 1280x900px (given the image can provide that size otherwise the size of the image itself).

Thanks for the response!

I'm not seeing any input fields for width/height in the plugin options. Am I looking in the wrong place (Options Tab > Plugin Tab > html_meta_tags), or do I need a newer version of the plugin? I'm using Zenphoto version 1.4.5.9

Your Zenphoto 1.4.5.9 is outdated and almost two years old. You cannot just update the plugin - that is not tested - you need to upgrade to the latest Zenphoto 1.4.13 instead. We can only support the current version.

Note that with 1.4.7 the version numbering changed a bit.