MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
To get a specified picture size you first have to crop the image to a resolution around 1600x1200 pixels with e.g. https://imageresize.org/.
Next upload it to http://3dp.rocks/lithophane/ with the settings from the "Instruction" section --> With the setting "maximum size" on 3dprocks you can determine the max length of the picture. The height of the picture is determined by the pixels.
If you want to control the dimensions of the image create a stl file with 3dprocks and upload it to https://www.viewstl.com/.
Hope that was helpful :)