Yeah we've gone over that dozens and dozens of times and there is no right answer - just a personal call....
6 cf, 13, 19, 30, 40 - they all use the same reg and accessories, and the price differences between the tanks are negligible really, so yeah why not get the biggest - or dual 80s?
For me, it was a question of size and weight, as I am packing mine in luggage for flights, humping the bags around airports and vans, carrying it all to & from boats, climbing ladders, etc. Varying some with brands...
13cf : 12.87" long & 5.94# plus valve, reg, etc
19cf: 18.56" long & 8.09# plus valve, reg, etc
30cf: 20" long & 13.7# plus valve, reg, etc
40cf: 24.9 " long & 15.9# plus valve, reg, etc
For me, the 19 gave me almost 50% more than the 13, or almost 1/4 of the usual 80cf for only 2# more, altho I do wish it wasn't as long. The 30 is 5.6# heavier, the 40 almost 8 pounds heavier and just excessive for bailout purposes to me.
Except that needs do vary, so while the 13 might work ok, I saw the 19 as my call.