rex is definitely the authority on plugins but to add to what i think your problem is that you are not passing the correct instance of the spritefont object your trying to change to the function you are calling.
"this.inst" <- I assume that's the instance of your plugin your passing to the function call? But what it needs to be is the sprite font object instance you want to scale.
You need to somewhere get the instance of the sprite font object usually by adding a condition to pick it. Once picked you store instance of the sprite font into a variable like "this.spriteFontInstance"
Your function call would then look like this.