We can find Unit Scale in the bottom right side of the Blender window. There we should select Scene tab, expand Units section and we will see Unit Scale value, which by default is 1. Also, make sure that the Unit System is set to Metric because UE4 uses metric system.

We changed this value, because UE4 one unit is equal to 1 cm, but in Blender default unit value is 1 meter, that's why we scaled it down to 1 cm. If you have exported models from Blender to UE4 before, you might think you didn't do this and all worked as expected to you, then probably you didn't used armatures in your models.

After Unit Scale change, there will occur a problem in a 3D viewport, that we cannot see objects from further distances. This happens because with changing Unit Scale also 3D view end clipping distance changes.

Clipping distance determines how far and near we can see objects from our viewport camera in our scene. That's why we need to increase it.