I used 3/32 last time (just a bit bigger than 5/64) - it was OK but a tiny bit too big. Would use 1/16 next time as I don't have 5/64. The spline will still go in the slot if you bang it with a mallet or hammer (you can use small piece of offcut wood, with a straight-but-not-rough edge, to spread the impact a little).
Watch some vids and do a test run if you haven't much experience with a router.