How do I check to see what size hard drive I have?

Right click (secondary click) the macintosh HD icon (could be named something else) in the top right hand corner, then click "get info". That should tell you what you need to know.

How can you not know how big the HD is though?

 

That'll only give you the size of the partition. Click on the Apple in the top left corner, select "About This Mac," and then "More Info." Scroll down to "Serial ATA" and it will tell you the formatted capacity along with every partition.

 

Apologies. I was presuming the HDD had not been partitioned as the OP didn't even know the size of it.