Make Chrome Extension Bigger

I recently made a chrome extension, but for some reason, when you first click it it appears very small.

My code:

#dif {
  width: 400px;
  height: 400px;
  font-size: 40px;
  color: black;
  text-align: center;
  font-weight: bold;
  padding: 100px;
  border: 5px solid black;
  border-radius: 20px;
  background-color: red;
  background-repeat: no-repeat;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

Dif is the name of the box that is supposed to appear.