Is there an optimum length for the handle and the fore grip on a 6 foot spinning rod? Or any length rod for that matter?

Does it change depending on the material, wood, EVA, or cork?

Any thoughts are appreciated.

It changes with application, line/blank weight and if it will be comfortable for the person using it. Also if it pleases you and your customers eye. Usually between 4 and 7 inches, I like a minimum of foregrip, just enough to comfortably ride your thumb on.

Spencer has it right, and I'll add it depends also on the arm length of the angler, and how he/she casts, works the bait and fights the fish. There is some trade off involved. Myself, I have short arms, and I like 7" of cork below a Fuji reel seat, and 2 1/2" of foregrip above, for my finesse rods. I work a weightless worm with my hand on the foregrip, forefinger just touching the blank above that. With those dimensions, the butt end is just short of the inside bend of my elbow when I work the lure, so the rod moves freely. For other applications, I might want a longer butt, especially if making long, two-handed casts is important. All this is a prime reason for custom rods.

Everyone likes something different.

IMHO - On store bought rods, I hold the rod and cut everything that extends 2" before my elbow. On customs, it's around 11-12" from top of fore grip to bottom of rod.

I'm doing a "sneaky pete" 3 pce six foot spinning rod at this time. 2 30" sections and a spigotted 15" butt. Light power, mod-fast action. On those light rods I go for 9 inches from butt cap to reel stem.