I thought i would chime in as Dan suggested,if you are looking at algos such as DES you will find that there are a lot of very similar algos which have the same sort of stucture called a Fiestel network which is used because of the mathematic properties of that kind of network,twofish,blowfish,aes, idea are examples of feistel networks,therefore they are all approx the same sort of size in terms of code.I am assuming that your data isnt of national importance so I would suggest that you take a look at a streamr based on linear feedback shift registers which will give medium security without taking huge amounts of code space.
google search gives a lot of hits on stream cipher source code