The Slice feature for sprites will soon be added to all Tileset Editors.

What exactly does it do?

Let's take a random sprite from the web and open it with Paint, or simply copy it.
We have a sprite of different size that is not optimized.

You can see that it is not centered, has a background that needs to be removed, and is not optimized for the editor we are using.
In this case, it is Rpgmaker XP.
First step: Remove the background and press the Fit and Scale button.

The sprite will automatically be resized while maintaining the proportions for the tool in question.

In order to have a sprite optimized for our tileset.
If it is still too large, simply press the Decrease Size button.
This will further reduce the size of the sprite while maintaining the appropriate proportions for the tileset.