10 gallon cooler will be plenty big for 90% of the recipes out there for 5 gallon batches.
Your tun needs to hold: ~1.4qts per pound of grain and figure the grain displaces .08 gallons per pound. Thats about 1.72qts per pound in displacement. A 10 gallon tun could do about a 17 pound grain bill to the tippy top.You could cut back to about 1.25 qts per pound if your grain bill gets big like that.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager