From experience - here is a public service announcement concerning proper sizing of the VMC Cross Over Rings (which VMC's web site has listed incorrectly (from my experience and opinion) :

 

Below are VMC's recommendations (along with my notes in parenthesis) :

 

Size Recommendations
3mm - 4" - 6" Finesse Worms, Robo Straight Tail   (*this is correct)
4mm - 4” Yamamoto Kut Tail, 4.8” Jackall Flick Shake  (* this is correct)
5mm – 5” – 6” Yamamoto Kut Tail, Yamamoto Cali-Roll, 5.8” Jackall Flick Shake (* use 4mm size for these applications ; 5mm for 5.8 FS)
6mm – 4” Yamamoto Senko, 6.8” Jackall Flick Shake, Zoom Trick Worm (*use 5mm size for these applications)
7mm – 5” Yamamoto Senko (* use 5mm for 5" Senko , 5" Yum Dinger , etc. standard 5" Stick Worms)
8mm – 6” Yamamoto Senko (for stick worms larger than 5" Senko use the 6mm size)

 

My favorite size VMC Cross Over Ring sizes are the 4mm and 5mm .

For Trick / Finesse worms save money and use Jig / Spinner Bait rubber skirt rings (BOSS Skirt O-Rings 50 pack)

 

Hope the above helps ! ... Comments are welcome ...

Agreed on the 5mm for 5" senkos wacky/neko rigged. Big fan. 

I did see a guy on YouTube who uses 6-7mm's for keeping his keitechs on jigheads better, and tearing less. Places it on the swimbait where the hook will come out, and then threads the swimbait on the jighead like normal. Punch the hook thru the crossover ring and the ring helps lock it in place I guess.