You can set the desired resolution by clicking on the ‘Options’ button located on the lower side of the Launcher Interface.

You can also change it in game by clicking on: Menu > Options > Settings > Resolution.

Finally, if you are looking for a specific resolution you can manually change it by opening the file rappelz_v1.opt, and by entering the desired Width and Height at the beginning of the file, before to launch the game.

You can find the rappelz_v1.opt in the Rappelz installation folder.