what is the source for Spanairds being only five feet tall?

I have no problem with a race of "giants" being up to say 10 feet tall. They would have had to be of different skeletal proportions (denser, thicker bones, stronger joints) than "normal people" as the rule concerning gravity and support comes into play above about 7.5 feet i.e. (basketball centers).

They would of course also had an interesting genome.

Beyond ten feet it seems to me that they would have had to be very different from us in skeletal proportions.