Make sure you work in the right Units (cm) and also export as cm. 1 Unreal Unit = 1 cm.
Make sure that your Reset your Transforms to 1 (I belive the Shortcut is ctrl + a)
This kind of errors usually originate from scaling. Imagine you import into Unreal with a scale of 2 but Unreal converts it to its original scale of 1 so it basicly halfs it in size. Its does it all across the Bord from Mesh to animation etc. If you got trouble to figure out test it on a Dummy Mesh like a Tube with 3 joints and see what is affected by what. It´s worthwile to understand the pipeline to avoid running into strange things again. Also worth noting is that Unreal does not understand Modifiers (there are to many across 3D modeling Apps) so make sure you bake them down and export without any Modifiers at all.
Also small disclaimer Im not a Blender user but the above applies to all Modeling Software the terms vary here and there.