This thread is resolved. Here is a description of the problem and solution.
How to Translate the "Size" string of a product attribute.
Go to WooCommerce -> WooCommerce Multilingual -> Attributes (tab) and there you should be able to translate the 'Size' string.
This is the technical support forum for WPML - the multilingual WordPress plugin.
This topic is split from https://wpml.org/forums/topic/string-translation-doesnt-works/