Bugatti Fan
A 54mm figure is the equivalent of 1/32nd scale...
That's not consistent across all figure manufacturers, though, mostly because 54mm wasn't and still isn't applied consistenly by figure makers. This goes back to the various makers of toy soldiers, before anyone made figures for mass consumption as historical miniatures.
Some makers reckoned 54mm from the top of a figure's head to the soles of its feet, ignoring any headgear the figure might wear. Other measured from the figures eyes to its feet.
Going back to the 50s and the emergence of figures sold as kits or castings and intended to be finished as detailed historical miniatures as opposed to toys, there are two makers who illustrate this difference.
One was Charles Stadden, whose figures were sold as 54mm, but they are slightly larger, and closer to 56mm, measuring from the top of a figure's bare head to the soles of his feet.
The other is Imrie-Risley, a collaboration between the artists Bill Imrie and Clyde Risley. Their figures are also sold as 54mm, but they are slightly smaller than Stadden figures. And I don't mean smaller, in the same way one man is 6' and another 5' 6". There is a slight difference in proportions. I/R figures are what some call a true 54mm, and as far as scale goes, they're 1/35. Stadden's figures are a big 1/32, closer to 1/30.
We see a similar inconsistency with 1/48 scale figures. Compare a Verlinden 1/48 USN pilot with Monogram's TBD pilot figure, and we see that the Monogram figure is larger. And we can compare that Monogram pilot with figures from earlier Monogram kits, and note that he's a little larger than they are, even though they're all 1/48 scale.
So you learn to develop an eye for judging size, and comparing it to scale, and you learn to see if two figures from different makers fit together. You learn to judge if two figures look natural, and therefore, look right together in the same setting.