First of all, you should have found a string in each wing panel to pull the wires through. If not, take a piece of string with a small nut or wheel collar tied to the end and drop it through the servo hole back toward the center it should drop through the holes in the ribs. Use it to pull the wires through.

A section of inner plastic pushrod will do the same thing, but you have to push it through. You can't push a string (at least, I can't...)

What length servo extension? Simple. The size that fits! Use a ruler and buy the one that fits the best.

I personally make my own extensions by cutting off the servo connector, soldering #22 stranded wires to the servo and cutting them to the exact length needed. The fewer connectors, the fewer the problems