How can you tell how much GB of hdd a motherboard can take?

Well, your motherboard supports IDE drives (which are essentially impossible to buy any more) and up to four SATA drives. There is no specific limit to the size of a SATA drive. The last I've seen they make them to hold two terabytes. So, if you want four of those, you can expect them to be just fine. (What you are going to do with four terabytes of space is another question.)