PFont font;

void setup() {
  size(500, 500);
  font = loadFont("SourceSansPro-Regular-50.vlw");
}

void draw() {
  background(255);
  
  fill(0);
  textFont(font);

  float fitWidth = mouseX;   // use the mouse as the "width"
  String text = "Rabbits";   // text we want to write
  
  // calculate the text width at full size (50px in this case)
  float baseWidth = textWidth(text);  
  float targetSize = fitWidth / baseWidth * font.getSize();  // size to fit
  
  textSize(targetSize);
  text(text, 0, height * 0.5);
}