Could anyone tell which one is the factory length?

 

Your best bet is to contact your nearest Lexus dealer, give them your VIN, and they should be able to tell you what your original part was.

 

I'm fairly certain 446.5 is stock from the factory. Maybe someone else can chime in to confirm.
Keep in mind, assuming you're trying to correct the camber, the 441.5 arm is only about 3/16" shorter and will likely only change the spec by 0.1
Have a conversation with the tech doing the alignment as it may not be enough to matter or it could be just what you need...