The parametric storage box with lid can be easily customized in Onshape by changing the parameters. The design is such that the wall thickness in the corners is the same as the normal walls, resulting is smooth printing. 

The included STLs have the dimensions as shown in the parameter image.
(AA battery in the print picture for size reference)

To be able to change the model, you do need to register on the Onshape website.
This is free and Onshape is nice browser based CAD software.

The Onshape link to the model can be found by downloading the text file.

Changing the parameters (see image) changes the model's size/shape.
The width/depth/height in the parameters are the box outside size (without the lid).
The lid fits over the box so the outside size is a bit larger (width + 2wall thickness + 2*tolerance, you can see the exact size in sketch 1).

There is also an optional easy to remove designed support for the hole in the box included.
If you like to have the support in the hole, select the parts Box & BoxHoleSupport and export them together.

You can export the parts after resizing them by right-clicking on the part and select export. (also see image)

Printing: When choosing thinner walls (<2-3mm), use filament with proper layer adhesion. Would not recommend silk, matt or any type of filled PLA in such case.