You size your tank for the highest fuel consumption race of the season(which may very well be Turkey) minus a little bit. Wirth's mistake was a whole different situation that he explained in an interview, cut the new team some slack.
It may have been that McL & RBR started the race with the tanks full to the brim and still had to conserve fuel to make the end... and we could see this again at Silverstone, Spa & Monza. Also depends on SC... none at Turkey put them really on the limit.
Why comprimise yourself at 18 races with too large a fuel tank when you comprimise yourself with 2-3 races with too small a fuel tank.