I'd say big enough to fill in clithes and not have creases in it like the chest caving in and flat back or straight shoulders, sleeves dangling, etc but still slim lookin with relatively low bodyfat.
Can't blame genetics for not growing. Certain muscles will be harder to grow because of genetics but doesn't mean it cant. Look at jujimufu. There are skinny ectomorphs who become big from good diet and years at the gym. Most black guys I knew from jnf were scrawny as hell back then and now they never even look like they were ever classified as being in the boney ribcage showfest category.