After the initial softmod process u don't really need an SD card, u can save apps on a fat32 usb.
An SD card is needed just for installing bootmii, making/restoring bootmii backups, or running sneek. Uneek can be launched without sd using usbloader gx's features
Syscheck can't dump logs to usb, but it's not an app you'll use regularly, and onscreen reporting is adequate.
If u WANT an SD, 1GB should be large enough for all the above (except maybe a large emunand on sd for sneek, but uneek/usb can be used instead anyway). Large SD cards are only really needed if not using a fat32 usb for games
There may be some other obscure apps that require SD (eg. Crazy intro video), but for the most part u don't need one after the initial softmod...