"Are these ones the big bulls?"
Translation:Bisi rōvyz vandissa issi?
In what instances are we supposed to use rōvi instead of rōvys, because both were used to describe "big"
In this case, bisi is a noun, specifically the plural of the noun form (substantive) bisy of the adjective bisa. In other words it is the noun "these (ones)", which is different from the adjective "these ...". http://wiki.dothraki.org/High_Valyrian_Adjectives#Substantives