The quick and easy way, assuming that you want the end document to *all*
be using the same font and size, is usually cmd-A to Select All, then
select a Font and Size via Format | Font or the Formatting Palette.
For the *best* way, per usual when dealing with Word, it all depends--on
what you want, and how the document is currently formatted, and what you
are going to do with the document in the future. You might do a save as
before the above, just in case you don't like the quick and easy way.