I've made a free online tool that allows you to create lithophanes that include a frame with customizable dimensions and can be curved to any amount you'd like. This will save you from having to design a frame or support and combine it with a curved lithophane in Meshmixer. If you see any way to make this tool better, please email me at email@example.com!
The scale of the curved lithophane comes from the lithophane height and the aspect ratio of the photo that you use. The Angle you select will determine the angle denoted in the picture (which is hard to describe with words, but here goes) that corresponds to the angle of the arc of the lithophane's transparent surface (sheesh!). Looking at the picture and varying the Angle with the tool should make it apparent. The radius of lithophane is calculated based upon the aspect ratio of your photo, the Lithophane Height that you select, and the Angle you select. Together, these variables set the major dimensions of the lithophane.
The frame's size is controlled with the base height, base width, and ledge angle parameters. And just to clarify what the schematic is showing (perhaps I should have explained this up front): on the left is what you would see if you were looking directly down onto the lithophane from a xy plane cut halfway up the lithophane, and on the right you see the lithophane from a plane cut in the xz plane. The ledge angle should be set to whatever angle you're comfortable printing for an overhang, because the ledge angle will be an overhang where the top frame goes to the lithophane image.
The maximum thickness and the minimum thickness refer to the maximum and minimum thicknesses of the lithophane image shell. You can use the maximum thickness and minimum thickness values to adjust the lithophane's brightness and contrast.
After you have selected your settings, simply upload your photo and out will pop an stl file that is ready for you to slice and 3d print without the need to stich multiple 3d models together.