It depends on your boil time. If its 60 minutes then you need just over 6 gallons to end up with 5 1/4 if its a 90 minute then you need just under 6.5.
I would say just go a little over 6, and then you might just need to top it off a little when you put it in the primary.
The volume would be
The radius of the pot squared times pi(3.1415...) times the height
(Radius)^2 x 3.1415... x (height)
There are 57.75 cubic inches in a quart so what ever you get from the equation above divided by 57.75 and thats how big it is in quarts.
As for cleaning it I would soak it...
So I want to make this one, but I'm only doing extract right now. So I plugged the original recipe into BeerSmith and converted it to extract and to do a 5 gallon batch. This is what I got:
3.3 oz Rice Extract Syrup (7.0 SRM) Extract
3 lbs 3.7 oz Pale Liquid Extract (8.0 SRM) Extract
1 lbs...