Hello @handmadehome ,
There is no default way to set a dropdown for product dimensions.
If you need to do this for a large number of products, you can use the CSV export-import method to update the dimensions in bulk.
Adding dropdowns in the dimension will require customization.
I’ll keep this thread open for a little longer to see if anyone else can chime in to share any other hints.
We have our developer resources portal that can help you get going, so I would recommend you to check it further from here: https://developer.woocommerce.com/
You can also visit the WooCommerce Community Forum, the WooCommerce FB group, or the #developers
channel of our Community Slack. We’re lucky to have a great community of open-source developers for WooCommerce, and many of our developers hang out there, too.
Another option is to check our customizations page to hire an expert that can create a custom solution for you: https://woocommerce.com/customizations/
Thank you.