it should be either
오렌지들'은' 대부분 작아요
대부분'의' 오렌지들'은' 작아요.
But since Koreans tend to omit plural form, it is more natural to simply say 오렌지 (orange) not 오렌지'들' (oranges). It is grammatically correct to not use plural form in this case.
오렌지+(-가) is used when 오렌지 is the active subject. This subject describes the property of the 오렌지, so 오렌지 is not doing anything. So 오렌지+(-는) is right. 오렌지+(-가) could be used in sentence like, "오렌지가 땅에 떨어졌다" (The orange fell to the ground).