Would you recommend a 20 or 30 size Abu Garcia reel for a basic drop shot/ned rig set up?  I'll probably put it on something 6'6" Medium action with 15 pound braid and a fluoro leader....

I'd go with the 20 if it's between those two - I'm using a Pflueger 25 for my dropshot/general finesse work...which is a little smaller than an Abu 10. For Neds, I usually use my Pflueger 20...which yes, is smaller still.

 

For finesse - I'm running a 7' ML/F rod, 6# Sufix Elite Mono. Neds are on a 6'6" UL/F rod, 4# Yo-Zuri Hybrid.

I would use a size 30 reel, unless the reel will only be used for dropshot. The 30 has numerous uses, dropshot,  neko, shakey, ned rig etc. 30 offers more versitility. Most of my spinning feels are 30's, including dropshot.