I tried using a longer spinning rod for shaky head fishing and didn't like it. I couldn't get it to balance right. Might have just been the one I was using, but I didn't like it. I ended up down-sizing a bit and now use a All Star ASR series - model ASR 825S. It's a 6'10" medium action rod with a fast tip. However, it appears that All Star isn't carrying that particular model any longer.
For that reason, I'm about to experiment with some new rods in the $80 price range. The Abu Garcia Vendetta rods feel pretty good. I just purchased 6'6" and 7'0" baitcasting rods in that series, and they balance very well. I haven't fished with them yet, but that is coming soon. I'm considering trying out two of their spinning rods for the purpose you mentioned - The VTS70-5, which is a 7' Medium spinning, and the VTS-74-5, which is a 7'4" Medium spinning. I haven't bought them yet, but when I do, if you are still looking, I'll give you an update on how I like them.
However, to answer your question - I like a rod "about" 7' with a medium action. I'm pretty sure I'll end up with that VTS70-5 for shaky heads this year.
All the best,
Glenn