What is the abstract noun for size?

The noun 'size' is an abstract noun as a Word for the relative extent of something, state of affairs as summarized (That's about the size of the situation.)

The noun 'size' is a concrete noun as a Word for the dimensions, proportions, or amount of something; a series of graduated measurements of clothing or objects.

The abstract noun form of the verb to 'size' is the gerund, sizing.