I have no idea. Ariane 5 rocket has payload dimensions of 15x50 feet. Starship is 26x56ft. The unfolded mirror is 21 feet across, so if you increased that by 73%, then 36 feet. This is all probably really unfounded assumptions
The benefit of starship here is probably being able to massively reduce the expense and risk of future JWST duplicates that don't need to fold. Also in the launch cost, which was $1 billion for Ariane 5 whereas Starship would be in the tens of millions.
