For those who haven't yet tinkered with changing 3D files, I strongly recommend using Tinkercad.com. If free and lets you do all the simple stuff very quickly and easily. Yeah its limited if you want to complex stuff, but most people don't.

When you download from makerworld, ensure you download the STL for Tinkercad (not the 3mf). Once you have the file in tinkercad, to alter the platform to any size, it should be a quick case of creating a basic rectangle of the size needed and moving it to the correct position. Then export and print to finish.