fiber optic size for Doodlebug maker lights

[hotrainlover]

Dear board,
I plan to use fiber optic strands to put lighting in my maker lights, on the Bachmann Doodlebug.  Has anyone tried this?  If so, what size hole/mm strand should I use ?  Thanks for the help. 
By the way, I found colored 1mm gen stones that are cheap, and self stick.  I purchased them at Hobby Lobby.
Thanks for the help!