Hi Richard!
Sorry for being silent for a while. We have been to IRCE. Now I am back from the trip and catching up with all the work.
I am not sure, but I feel that majority of people tend to use a custom theme. Anyways, if you feel that this setting is a must, please post an idea here:
http://ideas.x-cart.com/forums/229428-x-cart-5-ideas
We want to implement features that are needed for merchants/developers, so your voice would be highly appreciated.
Finally, if you need to change the thumbnail sizes in your own store, I can show you the approach as a part of dev docs. Now I am working on complete guide for theme creation, so I will describe this process there.
UPDATE: oops, I already explained it earlier in this thread:
http://forum.x-cart.com/showpost.php?p=373839&postcount=2
Tony.