I had one for a while I believe it's 2 gallon depends on media used and such. I currently have the fx4 and I believe it to be a far superior design and easier to clean and empty, if you're looking to buy one or upgrade.
You can find fx4 on sale for 200 and the fluval 406 for 150. Also I've got replacement gaskets if you need some for the 406 just pm and I'll ship them to you