Are you referring to the page size, the paper size or the screen zoom?

Page sizes for new documents are determined by Word's Normal template. If yours is anything other than the default Letter or A4 size, that means someone has edited or replaced Word's Normal template. The solution is to open Word's Normal template for editing and re-set its page size.

Paper sizes, naturally enough, are determined by whatever your current printer setup is configured for. If you've told the OS to use a label printer that only supports small label formats, that's all Word will show in Print Preview, for example. This can lead to situations where a document designed for a Letter or A4 size will only display a small portion of what the document contains.

The document zoom (on Windows versions, at least) can be controlled via a slider in the lower-right corner of the screen or via the zoom tools on the toolbar.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]