Welcome to the docs for the Texture Array Tools editor extension.
This is a small and lightweight asset currently focused on dealing with Texture2DArrays, that so far, as of Unity 2018.3, to quote the Unity docs directly, "do not have an import pipeline for them, and must be created from code, either at runtime or in editor scripts." Naturally, this proves rather frustrating when wanting to fiddle with textures currently bundled in an array, ergo why I made this.
Note that other assets with a similar function can be found on the asset store, and I should've probably checked before making this. Look around and check to see what you like – whilst I can't speak for others, this one is rudimentary. But hey, you can try it out and delete it if it doesn't work for you.
One of the two main modes of operation of the tool is the extraction of textures from a Texture2DArray .asset file.
The other main modes of operation of this tool is the merging of several textures into a single Texture2DArray .asset file.
As the tool itself tells you, textures must be of the same size. The tool will not attempt to verify if that's the case. If you give it textures of varying sizes, it'll presumably either not work properly or throw errors, depending on the order of the textures. Don't do it.