I use a Garmin ETrex 20 with a pirated (sorry) City Nav SD card. The upgraded memory covers every road and trail in the ConUS.
Prior to that, for 5 years I used a Garmin ETrex Venture Cx with the same SD card. That was the precursor to the current ETrex 20, and it eventually crapped out after half a decade of abuse.
I like the smaller sized units a lot. As noted, they are unobtrusive. Additionally, they are cheaper (to buy, and to replace if stolen or lost), they are more simple, and they require no wiring to the bike (AA batteries, good for about 24 hours on a charge). This also makes them easy to port over to another bike, or use for hiking, etc.
I should note that I use a GPS only to determine where I am, not to navigate. I use paper maps to navigate. I suppose if I was relying on the GPS to navigate I might want one that was larger.