SO I Just encountered this issue myself as the scale of the target mesh was causing issues as the held weapons would not sit in the hands as its a much larger humanoid and reimporting would not have helped because any animations would also need to be re-imported with the same new scale and that’s a lot of work, if you have a lot of animations
So this I what I did
once you have your target mesh scaled to the source mesh, make a note of the scale.
Then In your actor rescale your skeletal mesh-like so,
using the scale you made a note of from the retarget.
Seems to work ok for me, so I hope this helps someone else out 