3 by 2 pixels. Basically invisible. Useful if you want a gap with 3:2 ratio. Just write Blank.svg|<i>Horizontal length</i> in px, of course. This is just a vector version of the Blank.png, which was listed as the #1 candidate to be converted to SVG.
<div style="display:inline-block;padding:0 3px;background-color:#BDC"><a href="//commons.wikimedia.org/wiki/File:Blank.png" title="File:Blank.png"><b>Original: </b></a></div> <div style="display:inline-block;padding:3px 0;"><a href="//commons.wikimedia.org/wiki/User:Chlewey" title="User:Chlewey">Chlewey</a></div> <div style="display:inline-block;padding:0 3px;background-color:#CCF">Vector: </div> <div style="display:inline-block;"><a href="//commons.wikimedia.org/wiki/User:Timeroot" title="User:Timeroot">Timeroot</a></div>
Own work based on: <a href="//commons.wikimedia.org/wiki/File:Blank.png" title="File:Blank.png">Blank.png</a>)