I’m not sure if there’s a way to check if the player’s avatar is R6 or R15 outside of the game, but, if you can, you can use a custom StarterCharacter that uses R6 and apply their HumanoidDescription to it, or otherwise change the player’s character to an R6 model (by changing the Character property of the player) and do the same. It seems to apply the description as accurately it can, based on whatever the avatar is.
I’ll keep looking and edit this reply if I find a solid solution.