You should use a standard hard drive for storing movies and recordings since you won’t see any performance gain using an SSD for playing media files.
The higher the GB a HDD has, the better. But, the higher the rpm of the HDD and the louder it will be.
With the decrement of the HDD prices now, you will be able to get a decent hard drive, such as Western Digital Caviar Red with a very good price.
Red drives are specifically marketed to be used with NAS storages and they are good choice if you use your HTPC as a media server.
2-4 TB hard drives are currently optimal sizes in terms of storage and price. Green hard drives that run at slower 5400 RPM speed are a great choice as media hard drives.
Keep in mind that the more hard drives you use inside the HTPC case, the more noise and heat they will generate.