Sort by size option - how to do that?

[Marykity]

Hello everyone,
My joomla is 2.5.6
my vurtuemart is 2.0
The big question for me at the moment how to realize the sort by size option in my clothing online store. I use custom fields and create the child productr for different sizes but i have no idea how to do a sirt by size option on the category page where all products are available.
Ans the second minor issue - the user can all the child product to the chart more than available - how to deal with it?
Please help!!!

[PRO]

http://breakdesigns.net/extensions/custom-filters/