。
My book collection is twice the size of his.
Literal
I [of-の] book-collection [topic-は] his 2-times is.
蔵書 ('book collection, personal library') is a specific noun for one's collected books — more formal than 本. The comparison X倍 ('X times') is the standard way to express multiplicative ratios. 彼の here elides the noun 蔵書 — Japanese allows easy elision of the understood head noun.