It's not just paddle fitness, it's also regaining that ability to pop up as quickly as you can and when you do pop up, you need need to have that sense of balance that probably started to desert you 15000 schooners ago, luckily the 666 has probably helped you keep the balance thing good and strong.
Trust me, I've been there and in most respects, I'm still there

The wider the better, the longer the better, 24" x 10' min. Bigger if you can.
And don't ignore the soft tops where you're starting again, there are plenty of guys in my local club who, when it's really small and full, compete every month on soft tops, granted they,re the upmarket surftech soft tops, but soft tops nonetheless.