One click fix!

Change Resolution allows you to resize, edit and change the screen resolution of Java based JAR games to any of the following predefined formats : 176 X 208 (Basic) , 240 X 320 (QVGA) and 352 X 416 (Double) right from your desktop.

 Image Via Softonic

Just select the original and desired screen resolution for the game you want, select the JAR file from your computer and it will automatically make the necessary changes for you. Be sure to backup the original JAR-file first.

Click here to download

My take on it: 

Nice application, but the lack of lower resolution options is really a downside. This will make conversion much faster. Anyway, thanks to Macman for giving the link for it.