You can do this manually by hacking the XHTML / source, but this distorts the contents:

Steps to Reproduce

1. Create a new page
2. Add pptx attachment for test purposes (or you can attach test.pptx)
3. Specify the width to "40%" or add this storage format using Confluence Source Editor add-on to see different files:

   <div class="contentLayout" data-atlassian-layout="{&quot;name&quot;:&quot;pagelayout-three&quot;,&quot;columns&quot;:[&quot;&quot;,&quot;&quot;,&quot;&quot;],&quot;header&quot;:true,&quot;footer&quot;:true}">
   <div class="header">
   <div class="innerCell">
   <p>Height: empty<br />Width: empty&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; (Original)</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p></div></div>
   <div class="columnLayout threeColumns">
   <div class="cell ">
   <div class="innerCell">
   <p>Height: empty<br />Width: 100px</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="width">100px</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p>
   <p>&nbsp;</p>
   <p>Height: empty<br />Width: 40%</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="width">40%</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p>
   <p>&nbsp;</p></div></div>
   <div class="cell ">
   <div class="innerCell">
   <p>Height: 100px<br />Width: empty</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="height">100px</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p>
   <p>&nbsp;</p>
   <p>Height: 40%<br />Width: empty</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="height">40%</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p>
   <p>&nbsp;</p></div></div>
   <div class="cell">
   <div class="innerCell">
   <p>Height: 100px<br />Width: 100px</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="height">100px</ac:parameter><ac:parameter ac:name="width">100px</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p>
   <p>&nbsp;</p>
   <p>Height: 40%<br />Width: 40%</p>
   <p><ac:macro ac:name="viewppt"><ac:parameter ac:name="height">40%</ac:parameter><ac:parameter ac:name="width">40%</ac:parameter><ac:parameter ac:name="name">test.pptx</ac:parameter></ac:macro></p></div></div></div>
   <div class="footer">
   <div class="innerCell">
   <p>&nbsp;</p></div></div></div>

Expected Results

The file should be shown without any grey areas and should use the correct size

Actual Results

This is how it looks like when the macro is set with different sizes: