Creating a dynamically sized text overlay with iMagick

I am trying to create a text overlay on an image using php and iMagick. The crucial part is that I want the text size to scale to fill a predefined box, so I do not want to set a fixed font size. I have found the code at ImageMagick - Text into rectangle but need to adapt this code for the iMagick. I seem to be able to do everything except the crucial part -size ${width}x100

Here's the code I have, with a few things that I have tried commented out

$draw = new ImagickDraw();
$draw->setGravity (Imagick::GRAVITY_SOUTHEAST);
$draw->setFont('Bookman-DemiItalic');
//$draw->setFontSize( 30 ); // don't want to set a fixed font size
$draw->setTextUnderColor('#00000075'); 
//$draw->setSize(0, 0, 200, 200); // error: Call to undefined method ImagickDraw::setSize() 
//$draw->setViewBox(200, 200, 800, 800); // appears to do nothing
//$draw->rectangle(200,200,300,300); // this just draws a separate white box
$draw->annotation(100, 100, 'The quick brown fox jumps over the lazy dog');  // works, but it's a default (small) text size and the box simply fits the exact  text with no border
$img->drawImage($draw);

Is there a size option that somehow I've missed in the docs?