I use a 16gb and have it wired to the right front fairing glove box. I have an '09 SERG.
I tend to do playlists as artists. I will have as an example B.B. King as a play list and I'll fill the playlist in iTunes with my favorite B.B. King songs. I'll have playlists for Disco Classic and Cher.... Please don't judge me.
I also listen to Yngwie Malmsteen, Kenny Wayne Sheppard, ZZ Top, YES, 38 Special, Walter Trout Band, Vinnie Moore, Yardbirds, Bad Company, Bad Finger, Clapton, Black Crowes, The Band, Connie Francis, Santana, Robert Cray Band, SRV, Prince, Frank Sinatra, and the list goes on and on and on. The guys at the Pre-NMITM can attest to my eclectic music tastes.
Its easy to just scroll through the names as playlists. I also have classics of many years and I'll group the years like '60 - '61 - '62 - '63 as one playlist. then 4, 5, 6, 7 etc. It keeps me with around 40 to 50 songs in each playlist.
I will usually ride with the 16gb for a month or so and then swap out for another 16gb of new stuff. But I always keep a few special playlists for the wife that makes her feel good.
So a 16gb or 32gb at the most would be what I would recommend. Now that would be for the older 40 pin iPod modules. On the newer bikes with a USB, I would recommend a similar sized thumb drive loaded with your fav's.
Music is VERY personal and this is only what has worked for me.
Good luck with your music.