Do you mean where to buy from or what size?
Well wht ever size hdd you want and wht ever interface your motherboard has unless you want a PCI controller card.
My music used up all my HD space and I would like to increase it, possibly by purchasing another inner-HD, if that is even what it's called. What sizes do they come in?
i don't think that makes too much sense...
just buy another hard-drive. you can get any size you can pretty much think of...750GB enough?
you can buy them anywhere. a local store would be good, but internet is also good. search around.
HDs come in many sizes...probably too many for me to list right now. Just to list a few common ones, from newegg: (these may not be the best performance-wise; they are just for use as an example)
in the case that it's not SATA, then please don't buy any of those that I listed. Those are all SATA 3.0Gb/s, and aren't compatible. though, my brother has a WD 160GB ATA100 hard drive that's been working fine in his older dell.