Travel between Naoshima and Shikoku?

I'm thinking of going from the mainland to Naoshima and then on to Shikoku. I wonder if anyone knows what transport is available from Naoshima to Shikoku.

Ferry to Takamatsu:
https://www.japan-guide.com/e/e5476.html?aFROM=5400_5475
https://www.shikokukisen.com/en/instant/

Can anyone confirm if there are any baggage limitations when catching a ferry from Naoshima to Takamatsu? (Or extra cost).

There are two types of ferries: small passenger-only ferries and large car ferries.
I could not find any specifics on luggage policy, but I suspect that the policy on the large ferries is quite loose and you can bring a large suitcase without any problems, but it may be a little stricter on the smaller passenger boat.

https://benesse-artsite.jp/en/access/

This page is useful for ferry details. Couldn't find any info re baggage.

On the occassions when getting the larger ferries, even with groups with LOTS of luggage, never a problem. On the smaller fast ferries, space is much more at a premium as you essentially only have your seat and there is not dedicated luggage space. The last few years I have only been using the fast ferry from Takamatsu to Naoshima and the slower ferry for the return. Ages since I did a ferry to/from Uno.

I have seen people bring luggage on both the small passenger boats as well as the large ferries. During my numerous trips, there has been no luggage charge.

As has already been stated, there is limited space for luggage on the small passenger boats. It is much more convenient and easier to take the large ferries, as you don't need to worry about either passenger or luggage space.

By the way, although much closer to Okayama Prefecture on the main island, Honshu, technically Naoshima is considered a part of Shikoku's Kagawa Prefecture.