I'd have to say -1 for this, its extremely trivial to implement and better left as a utility class, also not every game will need this, most just have a fixed resolution that is easy to just hard code into the source.
Its a good feature and would be useful to have but maybe not in the core slick jar.
1. I think it should be part of the Slick core. If I know Slick correctly it mainly provides you with the necessary scaffolding for your games (input, window setup, game loop, states, etc.) and, primarily, ability to draw graphics in a similar way as Java2D.
Already we have pathfinding, tiled map support in the slick core. Why does setting up your game window belong to an extension and not those?
2. As you said, it's something that is very small, so doesn't add anything to the size of the library.
3. I can imagine many programmers would like to have this option in the core, and also people running the games, without the hazzle of downloading some extra jar for it or copy some java code from a forum post.
Anyway, just a request