Adjusting centered text font size based on image size. How?

With some research, I have created the following piece of code to generate a simple image with a text string right on the center showing the resolution of this image. The challenge is, when you change X & Y dimensions to larger or smaller values, the font size remains the same.

My question is, what would be a good way to "grow" or "shrink" the font size as the user chooses between image sizes?

One idea I had would be to calculate predetermined areas, add them to an array of key value pairs where the value is the hard-coded font size for a given range of areas.

Perhaps you have a simpler idea?

PS: The font I am using can be downloaded from Google fonts here:

Montserrat-SemiBold

The output looks like this:

And the code:

<?php

  // We Declare our canvas width & height dimensions
  $myCanvasWidth  = 256;
  $myCanvasHeight = 256;

  // We Create an empty and dark canvas from these dimensions above
  $myCanvas = imagecreate($myCanvasWidth, $myCanvasHeight) or die("Can't create image!");
  // We Allocate a color to be used as the canvas background
  $colorIndigo = imagecolorallocate($myCanvas, 0x3F, 0x51, 0xB5);
  // We Apply color as canvas background
  imagefill($myCanvas, 0, 0, $colorIndigo);

  // We Allocate a color to be used with the canvas text
  $colorWhite = imagecolorallocate($myCanvas, 0xFF, 0xFF, 0xFF);

  // We Declare our TTF font path in Windows 10...
  $myFont = 'J:\Montserrat-SemiBold.ttf';
  // We set the font size
  $myFontSize = 16;
  // We set the text angle
  $myTextAngle = 0;

  // We Declare the text string to be drawn on canvas...
  $myText = $myCanvasWidth . ' x ' . $myCanvasHeight;

  // We Calculate and return the bounding box in pixels for the text string to be drawn on canvas...
  $myTextBoundingBox = imageftbbox($myFontSize, $myTextAngle, $myFont, $myText);

  // Get the text upper, lower, left and right corner bounds of our text bounding box...
  $lower_left_x  = $myTextBoundingBox[0]; 
  $lower_left_y  = $myTextBoundingBox[1];
  $lower_right_x = $myTextBoundingBox[2];
  $lower_right_y = $myTextBoundingBox[3];
  $upper_right_x = $myTextBoundingBox[4];
  $upper_right_y = $myTextBoundingBox[5];
  $upper_left_x  = $myTextBoundingBox[6];
  $upper_left_y  = $myTextBoundingBox[7];

  // Get Text Width and Height
  $myTextWidth  =  $lower_right_x - $lower_left_x; //or  $upper_right_x - $upper_left_x
  $myTextHeight = $lower_right_y - $upper_right_y; //or  $lower_left_y - $upper_left_y

  //Get the starting position for centering
  $start_x_offset = ($myCanvasWidth - $myTextWidth) / 2;
  $start_y_offset = (($myCanvasHeight - $myTextHeight) + $myFontSize * 2) / 2;

  // Write text to the image using TrueType fonts
  imagettftext($myCanvas, $myFontSize, $myTextAngle, $start_x_offset, $start_y_offset, $colorWhite, $myFont, $myText);

  // Draw a horizontal dashed line for reference only
  imagedashedline($myCanvas, 0, $myCanvasHeight/2, $myCanvasWidth, $myCanvasHeight/2, $colorWhite);
  // Draw a vertical dashed line for reference only
  imagedashedline($myCanvas, $myCanvasWidth/2, 0, $myCanvasWidth/2, $myCanvasHeight, $colorWhite);

  // We se the correct http header for png images...
  header('Content-Type: image/png');

  // We Output a PNG image to either the browser or a file
  imagepng($myCanvas);
  // Free any memory associated with myCanvas; image. 
  imagedestroy($myCanvas);

?>