If it IS actually Buddha Belly Bamboo aka Bambusa tuldoides 'Ventricosa', it will grow up to 55 feet in height with culms about 2 1/2 inches thick. It is a 'clumper' so it will NOT spread aggressively. It will remain smaller if kept in a pot.

Now, having said that, if you want it to form or 'make' bellies, then it will need to be deprived of water, and keeping it in a pot, will help make a nice belly forming bamboo. Since you will be growing it from seed, don't deprive it of moisture until it is well established.

As far as growing it from seed, well, I will have to let someone else post an answer, as my experience is not extensive in that area. The seeds that I did plant, I just put in pots with regular potting soil, after soaking them overnight in water. I put them in mostly shade and had several come up...may have been luck, though.

Good Luck

Kt

Bamboo seeds are rarely available due to the fact that it takes decades to over a hundred years for most species to flower. They also do it globally over a couple of year period; I have not heard of any Buddha belly bamboo flowering. Lastly, bamboo seeds are typically about the size of a grain of rice, and tear-drop shaped with a pointy end. For all these reasons and from your description, I would be surprised if you had a Buddha belly seed...or even a bamboo seed at all. All I can say is: plant it shallowly in potting soil, keep it moist, give it a little bottom heat, and see what comes up.

Here are a few pictures from one online merchant.

Here is a link that might be useful: http://www.bamboogarden.com/Bambusa%20ventricosa.html