Hello everyone!
We have been using Zen to allow our customers to size our images all worked good until the Version : 1.5.1 upgrade
We uploaded 3000 x 3000 images to an album
Prior to this update I had the option for the user to pick a "radio button" resolution of 500 x500, 1000 x 1000, and 2000 x 2000.
Has this feature gone away or just configures in a new place?

You mean on the frontend I assume. Then you probably miss the viewer_size_image plugin which has been removed in Zenphoto 1.5.
https://www.zenphoto.org/news/zenphoto-1.5/#--removed-plugins

Reason is that it actually does not make sense to pick an image size in responsive web times (even despite that most included themes are still behind times for time reason).
The plugin is still available here:
https://github.com/zenphoto/unsupported-plugins-official

So I installed the plugin but still can’t get the sizing to work
I have the viewer_size_image settings as the following

Default size
$s=500

Image sizes allowed
$h=510; $w=510
$h=1000;$w=1000

Selector
Radio button

However the Radio buttons do not show up for the image selection

Depending on the theme user you have to add the plugin function printUserSizeImage() to your theme's image.php. Some official themes have it still like the basic theme but others might not and since it is not officially included anymore it may be removed, too.
It is also possible that it is not working (compatible) anymore of course. Any errors in the logs?

any chance i could get a zip of the old version to compare code? otherwise i have to scrap this viewer

Not sure which old version you mean exactly. The plugin has not been changed besides some internal paths since it got unsupported in 1.5. It of course now needs to be installed in /plugins being not included anymore and not in zp-core/zp-extensions.

Older versions are included in older versions you can get via our download page (link on the right or directly on Github).