The reader built into the RT and the Pro models is designed for 64GB capacity maximum. The capacity limit is based on the reader/controller that is in the computer.
There are cards coming out going up to 2 TerraBytes in size but have not seen those for sale yet. The readers in these tablets will not recognize anything larger than 64GB.
The format for all cards should be either NTFS or Fat32. Nothing else or you can destroy the card and it will be useless. I already ruined a USB pendrive by using exFat by accident and ended up having to throw it away.
NOW, since these tablet do have USB ports, when the TerraByte sized cards are available, you should be able to get a card reader and use them. So, you won't be stuck with a 64GB limit with just the built in card reader.
An engineer in Singapore found out that if regular table salt was added to the mix that you could increase the storage capacity by 100 fold. That is where the TerraByte sized drives and cards are coming from. So far, they have been proven to be reliable in
the hard disk world and the same should be in the card/pendrives as well.