I dont have an exact answer but .. Non tempered and 4 to 6 inch wide x half inch thickness. For tempered, I would go wider due to shattering if it twists.. non tempered will break but wont throw shards in every direction. Are you thinking single brace or multiple.? I am guessing you are not talking Euro brace..
Single I would go a little wider . If multiple you can be narrower..
I see, after re-reading you want multiple. I would look at 3 Braces for a tank that long at least and some of that depends on the thickness of your tank glass.
Umm trying to think of someone we can signal to this post that makes tanks..