As far as the size goes, just use the same size fiber as the size of the eyes on the bait. you might have to do a little converting, (metric to standard, or vice versa) Just as was mentioned above, you need to drill a hole through the lure. This can be a problem in plastic lures, as the ballast weight or other obstructions are between the eyes causing the lure to bust or break when you attempt to drill. (this might explain why you see mainly rapalas with these eyes) I tried the plastic, but couldn't afford to keep breaking them till I got one right. So, I decided to use them in my handmade baits. This is how I did it: 1. cut your profile from your stock. 2. Drill a hole completely through the bait where the eyes would be.(same size as whatever fiber you plan to use.)3. roughly shape your lure like you normally would. 4. slide the fiber through the eye hole to get an approximate length.5. cut the fiber to desired length ( slightly longer than you need) 6. Place the cut piece of fiber in the lure and and or grind to match the conture of the bait (I use a dremel with a small sanding drum) 7.finish sanding the bait smooth, then prime or seal. ( NOTE: You may want to seal the bait without the fiber so the eye socket gets sealed also) 8. The final painting and clear is up to you. You can leave the fiber in and mask it, or paint with the fiber out and put the fiber in afterwards. you will have to play around with the fiber to get the desired effect.
This is how I did it but I'm sure there are many other ways. As soon as I make another one I will take pics as I go and try to put together a tuturial.
Hope this helps a little:)
Jay