Depends on your needs. If like me you just want to install a good text editor such as nano, it takes very little space. But if you intend to install Apache or Asterisk and start hosting a lot of content, you will need a larger card.
Personally I have an old unused 4 GB microSD in my RT-N66U (which is dedicated to FW development), and I only use about 11 MB for Optware (nano and all the required libraries). I also have a test/development copy of the webui in there, which takes a few more megs.
If you want to play more with Optware, I would recommend setting up Entware instead. It has a lot more programs available, and is actively maintained so you will also have much more recent versions of the programs that are available in Optware. Note however that it might not play nice with Optware, so I wouldn't recommend using Entware if you also want to use Download Master.