Hello,
Yes, you can use the sizes from the existing image or use the size from the new image.
This is the phpdoc of the size options of the method:
* 'width' (mixed): the value in cm (float) or 'auto' (use image size), 0 to not change the previous size
* 'height' (mixed): the value in cm (float) or 'auto' (use image size), 0 to not change the previous size
* 'dpi' (int): dots per inch. This parameter is only taken into account if width or height are set to auto.
You can set width and height values as 0 to keep the previous sizes or set new values (fixed or auto). Please note that if the images doesn't have the same dpi you may need to set a dpi.
Regards.