What is the largest file size that Pages can handle?

Follow Tom Gewecke's advice.

Only the Apple Pages Product team knows what the maximum, practical file size limit is for Pages v8.2.1, and they do not participate here. Even if you could import that 1GB Word document into Pages, you would create a continuous spinning beach ball instead of a useful application — because Pages abhors large documents, far smaller than what you are attempting to open.

Word is designed for large document, commercial usage, and Pages is designed for small document, consumer usage. It is also not a Word clone — placing document translation limitations on the integrity of the opened, and exported content.

Is there some special reason you want to import this into Pages? You are probably better off working with this book in Word or LibreOffice.