Substituting barley for pasta

The pasta in the soup is only there to add substance and calories, so the amount is not really fixed. You can change it according to your preference.

Without any further information about how you like your soup, I'd say go for a volume replacement, and add 1/2 cup of barley. When you pour a spoon of soup into your mouth, it is mostly the volume of the barley which you perceive as "amount" in relation to the volume of the vegetables and liquid. So if you were to start consider densities and try replacing by weight, the final soup will appear to have less starch than the original.

This is only a starting point, if you find out that you don't like the amount, you can always adjust it for the next batch to whatever you like it to be, without fearing interactions with the rest of the soup.