I would say, why not just encrypt & sign using GPG as usual... then put this into your steganography program of choice. That is... unless you want some security by obscurity... but then again, you're already doing some of this by using steganography.
There are many steganography tools available, and many different file formats to choose for embedding your message (ie: png, jpeg, tiff, mp3, avi, mpeg). A list of steganography tools can be found here. The size of the message, or how 'heavy' it can be depends highly on what tool you are using, and the size of the carrier file.
You can hide a lot more data in a larger file without it degrading the quality, or becoming noticeable.
For some simple open source steganography tools, perhaps try out OpenStego or StegHide
EDIT: Do note that depending on the context of the message transmission, a certain file type or a large file may indeed be very noticeable. For example, on a network where small images and HTML pages are predominantly transferred, perhaps a large DVD .vob would stick out like a sore thumb. The purpose of steganography is to conceal the message so it's completely undetectable. Context is everything here.