I am by far no expert on Switch or Spey rods, but here are the two I have built....... so far
. Top one is my St. Croix Imperial 5 wt., 11' The bottom is my newly finished Sage "One" 7 wt., 11'6". Both the foregrips are between 10 1/2" and 11", the aft handles are between 4" and 4 1/4". Why those dimensions? Just the way the cork patterns worked out. Shaped them the way they felt good to the hand.
I built the handels on mandrels. Don't trust my shaping skills enough to build the handle on the rod. Plus the thru hole on my Sherline is only 3/8" in diameter. The Sage definately wouldn't fit thru it.