IMO bean size wont tell you anything--both Arabica and Robusta come in a wide range of size gradings.
A good Robusta can add to a blend, and there are those who like Robusta unblended. It is usually higher in caffeine than Arabica.
Ive tried some Robusta in the few blends I drink and can take it or leave it.
In the end, if you like the coffee--drink it. ;)