▲最佳答案应该是 by。by 在此表示依据，意思是“根据”“依据”。by size 意思是“按大小”“按尺寸”等。又如：

The containers are graded by size. 容器按尺寸分类。

The fruit is washed and then graded by size. 水果洗净后按大小分等级。

▲in size 的意思是“在大小方面”。如：

Those shoes are just right in size. 那鞋大小正合适。

They differ in size but not in kind. 它们大小不一但本质相同。

It’s smaller in size but greater in weight. 它体积比较小，但分量比较重。

The models differ in size and shape. 模型的大小和形状都不一样。

The two towns are very much alike in size and population. 这两个城镇在规模和人口方面都十分相似。