My pit's chamber is ~40" and my plates are 1/4"T and 4"W. They work well. Anything in the range of dimensions mentioned will work. If you can get some thicker stock that will work too - plus it's adding more heat sink material to the pit as a bonus. Use whatever you can find cheapest and easiest. Don't worry whether it's cold roll or hot roll. Just clean any petroleum oil off of 'em and season with veggie oil.
OK Joe Longhorn & Original Ok Joe Vertical Roasting Oven hybrid, Char Griller Akorn, Falcon MK V gaseous grill