the size=尺码,尺寸.(指比较精确的数字)
How big=大小.(通常用来泛指一个物件的大小)
What is the size of the shirt?这件衬衫是甚麼尺码?
-->也可以说:(How big is the shirt?这件衬衫有多大?)
This shirt is your size.这件衬衫是你的尺码.
What is your size?你(穿衣服)的尺码是多大?
-->也可以说:(How big do you need?你需要的尺寸是多大?)
I will size up the shirt.我要估算一下这件衬衫的大小.
以上供您参考,