I a related post PM Module post, it was explained how to adjust the browser window size to match the form.

This post shows how to custom scale the width of the form itself. In my case, the message entry form was scaling itself to the width of the main page which was TOO wide.

Open PM/templates/pm-lite.html

Change the width100 spec to widthXX (60 seemed about optimal). This width is inherited from xoops.css so your width options you can enter here start at line 170 of xoops.css. If you specify a size that isn't there it will default to 100%. You can insert a custom size into xoops.css by copy/paste/modify (I created 55% between 50 and 60 to test and it worked).

Once you have changed the width (I settled on 60) in pm-lite.html, the size just doesn't change, the templates have to be recompiled. To do this, delete the PM smarty compile templates for all themes, clear the browser cache, go into modules administration, and update the pm module. This recompiles the templates to your new width.

When you open the send message window, you should now see the new size. Then readjust the browser window size for best fit using my related post.