There are two possible reasons for this. One, if you don’t have a VRPlayerController any hmd rotation may desync your rigged hands since your controller orientation is not decoupled from your hmd. The second is that the rigging uses compromises to match a mesh to your physical dimensions. It resizes the mesh arm length to match yours and this causes the hand to shrink, if this wasn’t the case the hands would deviate more. If you want a more 1:1 rigged experience try the floating LeapEchoRiggedCharacter, which without needing to adjust elbows and such the rigging is able to match closer to the debug hands.
If you can personally modify the rigging to have a better compromise, by all means make a pull request 
If you have a Mac, you should be able to just compile the plugin. You have full source and should be able to recompile it for 4.9 without issues. That said I’ll include a final 4.9 mac binary when image hands make it in to the plugin (Mac isn’t my main platform and I have an ancient Mac so unless someone takes up the Mac compile mantle it will always lag significantly behind the windows releases).
Yes, all you do is follow the instructions in the repo (and earlier in this thread) found under the How to use it - Blueprint without Convenience Content, Quick Setup heading. There you can forward/store all the latest leap motion tracking points to where you want to use them. You can also look into the LeapBasicRiggedCharacter blueprint to see how the more complicated rigging is handled using the intermediate AnimBody. Note that if you do not use the convenience characters, you will have to set your hmd mode from the component manually to be triggered when the hmd gets enabled (Optimize for HMD function).