hello
If it is a transform problem then push “N” in blender to bring up your transform menu. There you can see Location/Rotation/Scale and Dimension. Note that if your scale is not 1 on the XYZ axis you will get a transform error. Same goes for rotation which I try to keep 0. If it is different select both your Skeleton and your mesh push CTRL+A and select APPLY- ROTATION&SCALE.
Sometimes your mesh scale is 1.2 and your skeleton is 1(just an example) then you get an error. Are you using a custom character with your own animation or are you trying to modify unreal animations/character?
Plus update to Blender 2.75a they have improved FBX exporting tremendously.