I'm trying to adjust the thumbnail size for my k2 items but none of the settings seem to have any effect.

I set Thumbnail viewport width to 400 and Thumbnail viewport height to 300 and Fit Thumbnails to available viewport to "No".  The viewport size has increased but the thumbnails are still displaying the same size within the much larger viewport.

You can see the problem at:
wardnet.com/dev/rustonrv/index.php?option=com_k2&view=item&id=1:freedom-express-290bhs

I assume there is a setting for this somewhere that I am missing.  Thanks in advance for any help.

Switched from JQuery to Mootools and the problem seems to have corrected itself.  Very odd.

Sorry this is not solved.  The very next item I edited did the same thing.  You can see the tiny thumbnail at:
wardnet.com/dev/rustonrv/index.php?option=com_k2&view=item&id=5:shadow-cruiser-19h&Itemid=2

The original image is 600px wide which you can see if you click on the thumbnail.  None of my settings have changed since my earlier post.  I tried changing popup engine again but that has no effect.  It seems that has nothing to do with this fixing itself previously.

Any thoughts what is causing this?

Again the image has "fixed" itself.  This time with no interaction from me at all.  I suspect there is some caching going on in SIG Pro that does not get cleared when all caches are purged through the Joomla admin interface.

Did you clear your joomla cache?
In addition, go to the SIG PRO plugin settings, and follow the instructions are stated at the hover text of the Thumbnail cache expiration time option.

www.wardnet.com/dev/rustonrv/index.php?option=com_k2&view=item&id=22:viking-2107&Itemid=2

Same problem again.  Happens everytime I upload a new gallery.  Are you suggesting that the correct procedure is to go and set the cache to 0 everytime I upload something new then set it back to something like 120?  Why does it even set the thumbnail to the incorrect size in the first place?  Why isn't it generating the thumbnail size I requested when I first create the gallery.  This has to be a bug.

You need to set your cache to 0 (when you add a gallery) in order to see the change immediately.

Yes, when you set the cache to 0 then it will enable the system so that you can see the changes. I struggled with this for a while and couldn't figure it out for the longest time. Not sure if it is a bug or just the way they designed it.

This problem persists.  It is not a cache issue as the initial thumbnail is generated and posted in the incorrect size.  When I clear the cache it fixes the issue but I should not have to clear the cache everytime I create a new image gallery.  Somewhere in the code it is using a default image size instead of the one I specified in my settings and using that as the thumbnail.

Which K2 version do you use?