要看代词具体指代什么.
欢迎追问例如呢What size suitcase do you own?What size do you want? Size 36.What's the size of your gloves?这两个句型都正确对于你标题上的what size are they/is it和what's your size可以互换吗比如去买鞋都可以指代，因为都清楚问的是什么。what size are your shoes?what's your size?这两问都对。