How to set size of SVG image in :before pseudo-class?

You can use svg as background-image:

.withimage:before {
  content: "";
  display:block;
  height:125px;
  width:125px;
  background-size: 125px 125px;
  background-image: url(test.svg);
  background-repeat: no-repeat;
}

Here is example

Also you can try use this one:

.withimage:before {
  content: url("data:image/svg+xml; utf8, <svg.. code here</svg>");
    display:block;
    height:20px;
    width:20px;
}