Default in UE is 1 unit = 1cm. This is referred to as a real world measurement.
A Unit is not a true measurement but a means to have a set value that is true in any application being used. A Unit is the same set value in Maya, 3ds Max, Blender or even UE4.
A box made in Max 100X100X100 units would be expected to be the same “if” you are only using units as the means of measurement and only changes when you apply a true form of measurement relative to other objects with in the same scene.
In 3ds Max there are two two ways to manage measurement requirements.
System Unit Setup.
This is where you tell world space how big objects are to one another based on their relative scale using units as the base reference. If you have your units set to 1 unit = 1 inch you can then measure all things relative to world space. If you change this to 1 unit to 1 meter then everything becomes much bigger but should still be relative to real world measurements. To keep your sanity I would highly recommend that you set this to 1 unit = 1cm as this is the world setting that the majority of all 3d applications are now using and if you change this your going to run into some serious scaling problems.
Display Unit Setup.
In a nut shell this is where you select the type of measuring tape you would use as to how big objects relative in size are to one another relating to how world space sees them. Most get confused by this thinking it changes world scaling but all it does is swaps out the measuring tape.
The key here though is UE4 now uses real wold scale. You measure a door frame it does not really mater what your world scale is set to as long as it’s equal as to true world scale and if you brought an object into UE based on units of measurement and it changes then you really have a problem.