Our most generic file size that we've been using since our inception is 
2800 wide x 3200 tall @ 200 dpi.
This is a rectangular file and translates to 14" x 16".

This file size is used, and often recommended, because art that is saved at those dimensions can be applied to almost all of our products.

However, in the product mock up section you will likely find a highlighted Recommended File Size to be different from the 2800x3200 file size. 

This is only a recommendation that is catered to that specific product.

The 2800x3200 file size will generally work fine for most products. 

You can use the recommended file size if you'd like to hone in on the print area of a specific product (e.g., mugs (front & back prints) have a print area of roughly 3" x 3"; the recommended file size for mugs is 870 x 825 and 300dpi (which is more of a square format than the rectangular 2800 x 3200 dimensions)).

Regarding any potential empty space within the 2800 x 3200 px file:

-It's best to create your file at 2800 px wide x 3200 px tall @ 200 dpi (300 dpi also works fine) and place your design in that file space.

-Enlarge your art as much as possible to fill that space (of 2800 x 3200) -- it's fine if part of the file space is empty (for example if your design is more square than rectangular).

-Save as a PNG and use that file for various products. (We require PNG files because they allow for transparency).