If the data is important, then the card should also be important. I always stick with the major brands Pny, SanDisk, Lexar, etc.
As for size (from the FAQ):
The BlackBerry devices that have media card capabilities are from the 81xx, 83xx, 88xx and 9000 series.
All of these series use a MicroSD card and have the following maximum capacities:
Media Card Size Limit
BlackBerry OS 4.2.0 -- Up to 2 GB
BlackBerry OS 4.2.1 -- Up to 4 GB
BlackBerry OS 4.2.2 -- Up to 4 GB
BlackBerry OS 4.3.0 -- Up to 8 GB
BlackBerry OS 4.5.0 -- Up to 8 GB
BlackBerry OS 126.96.36.199 and above -- Up to 16 GB
BlackBerry OS 4.6.0 -- Up to 32 GB