In your enquiry you ask about hte use of "much of."
The sentence," The two shirts are much of a size,"does not make any grammatical sense. In Fact, I find it hard to understand the meaning. I can assume, as you do, that it means they are both similar in size.
In order to use "much of" you would need to use a non-countable noun in a sentence such as,"much of the information we gave them was young"
Regards