What is the best way to re-size a User Manual in Framemaker?

Creating a new empty document, then importing formats and flow might be the least troublesome method. At the very least it allows you to have the source and destination docs open at the same time for visual comparison.

If you do it in-place, some content could flow out of sight. If the source document has ill-considered layout (such as disconnected pages and frames), it might be unfixable in place.

The biggest problems with re-sizing are anchored frames, tables and tabs that make assumptions about column dimensions. At least in your case you aren't changing page width, so tabs aren't likely to be an issue.