How To Create a canvas with proportional size choices?

[mcmxmas]

How To Create a canvas with proportional size choices? I want to offer my wall art in multiple sizes but I want it to crop the image consistently - Proportionally. Not a choice of willy nilly sizes. Can I lock the proportions on the canvas sizes? What am I missing here? 

Here is the first piece published. It is a panorama size.- I don't want my customer just cropping it where-ever. https://www.zazzle.com/mid_century_modern_graphic_canvas_print-256856538329028471

Thanks in advance! Cheers.

[waterart]

I'm not sure if you can lock the canvas the way you want but giving customers more choices increases your chance of selling