setSymbolSizeExpression(expression) {
    this.hasSymbol_ = true;
    this.symbolSizeExpression_ = expression;
    return this;
  }
Sets an expression to compute the size of the shape.
This expression can use all the uniforms and attributes available
in the vertex shader, and should evaluate to a `vec2` value.
@param {string} expression Size expression
@return {ShaderBuilder} the builder object