It's actually quite simple - and there are many posts describing this, including some in this forum.

What you can do is to insert a single-celled table into the document with a fixed row height and column width. Any picture you insert into such a table will be constrained to those dimensions. This is particularly useful where you have many pictures to insert that have to be constrained to the same maximum dimensions. Simply make a multi-celled table with the # of fixed-dimension cells required, or make multiple copies of the same single-celled table, beforehand.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]

I can fix the width of a table cell to 4cm but I can find no way to fix the width.