Currently you can only use module size to specify the actual display size of QrCode.
Number of modules for width will be matrixWidth + 2 * QuietZoneModules.
Pixel width will be number of modules for width times module size.
Above Measure method is mess, It's from original demo which I should replace with newest code. Proper one you can find under WPF version of renderer class.
matrixWidth is determine by input text.
Create a image then resize probably is more accuracy than draw according to specific size. Easiest way probably will be HTML's stretch function.
<img src="image.jpg" width="200" height="200">
That will display any image with size 200x200.
Just make sure to give proper module size to renderer. Module size too big will cause image size way too large. Module size too small, stretch hard out, small QrCode will display poorly.