You should export everything at the same unit system from your modelling software. Just to make everything proportional. It also might depend on this software, as some have a different system. If i import Blender in C4D the models can be rediculous large or small. The software probably offers a independent unit, just as THREE which will be the same.
As mentioned i use a 1 unit 1 centimeter world scale, just cause of the precision issues that will appear on some mobile at larger distances. But i also use a coordinate system that is independent of larger numbers beyond 32 or 64 bit, so if you use smaller units as 1 you could hit the upper limit ealier if that is relevant to you at all.
Export a cube at 1 cm or 1 meter, by just looking in the model file (text-format), if the cube has coordinates like -0.5, 0.5 etc., the cube will have a size of exactly 1 unit. Most software uses the international standard you probably mentioned (1 = 1 Meter)