Mike, thanks for the friendly advise. Many of these shop toys can ruin a day in an instant. I'd bet a good quality 36 grit can do a surprising ammount of work, and starting there is a good way to have more fresh 60's around. jll, I'd ballpark 8 by 18, but consider going bigger than the minimum if you like a little work space and build to minimize vibration/deflection.
Take care and Happy New Year, Craig
Take care and Happy New Year, Craig