From Garmin FAQ

The Edge 800 can accept up to a 32 GB (gigabyte) microSD High-Capacity (microSDHC) data card. However, depending on the amount of information on the card, it can adversly impact the speed of the device.

Provided your Edge 605 / 705 is up-to-date your device will support a 4GB microSD High-Capacity (microSDHC) data card. To ensure your device is up-to-date, download and run our WebUpdater program.