Older Garmin devices (such as the Vista/Legend Hcx or GPSMap60Csx) will usually support up to 8GB cards, however they should also support cards up to 32GB (with the latest firmware installed).
All MicroSD cards MUST be formatted as FAT32 (for smaller cards 2GB or less you should be able to use FAT or FAT16). Using NTFS or HPFS will NOT work, as Garmin devices only support FAT/FAT32.
However, you may be able to Partition a 64GB, 128GB or 256GB MicroSD card to create a 32GB Partition on it (format it as FAT32), this may then work in your Garmin. This is an advanced process and NOT recommended.
[*] There are a few new models that do officially support cards larger than 32GB, these include the Tread (all Models , up to 256GB) and Zumo XT (up to 256GB). It may be that garmin will enable native support for larger cards in firmware updates for older models (or not).