Till today I never ran Vista ratings LOL.
I get a 5.0 mainly bc of my X2 4400+ but my HD rates a 5.2 and its a 2mb cache PATA HD 6 year old seagate still represents. Quality HD from my first PC.
I believe Windows goes off the disk transfer speed by performing a few tests. So you may notice an increase of speed if you got a faster drive, especially when transferring files and loading Windows. I believe a 5.4 drive will have a transfer of around 60Mb/s where a 5.9 drive (well in my case) is 104Mb/s
Not usually, the size is irrelevant in the ratings (I wish it wasn't), as on my college computer, I partitioned a 80GB hard drive with 30GB for the C:\ partition, and it still rates the drive a 5.7.