I agree that First Legion is the closest of the 1/30 scale manufacturers figures to a range of 54-57MM. Older Figarti figures were 1/32 scale before they increased figure size to be competitive with the 1/30 scale companys. Most 1/30 scale figures from K&C, TCS, Figarti and Thomas Gunn are about 1/28 scale and average 68MM in height from the top of the base to the top of the head; not including the helmet. First Legion are a true 1/30 scale with figures averaging about 60MM tall. However they can vary considerably between lines. Medieval figures seem to run larger for some reason!
Britains used to be 1/32 scale and 54MM tall; however some these figures have also grown over the years too. The best advise is to remember that human figures vary in size. WWII photos show soldiers ranging in height from 5'5" to 6'5" standing next to tanks with a known deck heigt of 6' as a reference. Figure proportion is more important to realism than absolute height of the figure. I buy what I like and if needed I can remove the base to reduce the height of the figure or build it up to increase the height. I use some of the larger figures from K&C and TCS with 1/25 scale Tamiya armour and they look very realistic in dioramas. I hope the forgoing helps. Regards