This is a processing sketch that generates Eagle BRD files for a heated bed based on parameters you specify. You can make it whatever size you want and use trace widths and spacing as needed to get whatever resistance you want.
THIS HAS NOT BEEN TESTED! I suggest making a small cheap board with an easy to measure resistance like 50 Ohms or so, ordering it through a cheap service like BatchPCB, and verifying that the calculations were correct! Throw on a cool design and you can make coasters out of your example boards. :)
This sketch uses Processing 2.0 (not 1.5, though it could be modified for that - look at the XML handling) to generate a native eagle BRD file using their new XML format, so everything is nice and clean.
Generate a board file, look at the Processing output window to see what the resistance, current, and power draw will be based on your input voltage (I can't guarantee those numbers are correct! I haven't tested this yet and I admit I wrote it pretty quickly - its more of an example. You should double check the length calculation section of the code - god forbid I messed something up and you waste money on a bad run of boards!)
The files are hosted on Github, so check there for any updates:
Download the zip from here, or get it all from Github. Read the Readme from the zip or from Github.
Also attached here are the example files of a fake "finished" board with an LED and some power connectors and mounting holes added. After generating the board it was trivial to make it into a real useful example.