Tallinn to Helsinki to Stockholm ferry

Try publishing this in the sweden forum. You will get more replies there. After having waited a week you can then try the General Europe forum.

Silja/Tallink- the two routes use different terminals in Helsinki, both of which have luggage storage- https://www.tallink.com/good-to-know/ports-and-terminals

You can take as much as you can handle on board- only so much fits in the cabin, but each ship has (differing) luggage rooms for the remainder- https://www.tallink.com/good-to-know/luggage

Viking use another (3rd) terminal to either of the Silja/Tallink terminals- https://helsinki-tallinn.com/helsinki-port/viking-line-terminal/

On board luggage storage by ship (your browser should translate)- https://www.vikingline.fi/sv-fi/hitta-resa/bra-att-veta/bagagekontroll/

Both lines- you should always be able to book on the day, but better to book ahead, the further the better to avoid dynamic pricing, like the airlines

1) Are big luggage allowed on board? (those standard checked luggage for flight)

Yes

2) Is the terminal the same in Helsinki for both routes?

No. Silja and Viking use different terminals. .
The Tallink/Silja ships between Helsinki and Stockholm use the Olympia terminal in Helsinki
The Tallink/Silja ships between Tallinn and Helsinki use the Helsinki West Terminal 2 (Länsiterminaali 2)
The info on the Tallink ports is shown on their website: https://www.tallink.com/good-to-know/ports-and-terminals
Viking uses the Katajanokka terminal. Information is here: https://www.sales.vikingline.com/my-booking/good-to-know/harbours/helsinki/

3) Any lockers for luggage at the Helsinki terminal?

Both websites indicate the terminals have luggage storage (see the links above) and the port of helsinki website lists the prices for luggage storage (see links below). I’ve never used the luggage storage in any of the terminals. I have used the luggage lockers available in the Helsinki traain station.

https://www.portofhelsinki.fi/en/passengers/passenger-terminals/katajanokka-terminal/other-services-katajanokka-terminal/

https://www.portofhelsinki.fi/en/passengers/passenger-terminals/olympia-terminal/other-services-olympia-terminal/

https://www.portofhelsinki.fi/en/passengers/passenger-terminals/west-terminal-2/other-services-west-terminal-2/

4) Do we need to book these rides in advance? How far from departure to book?

Yes. The price will usually be lower if you book in advance and it is possible for journeys to sell out. Book as soon as your travel plans are firm.

5) Any advantage to book both routes with the same company? (Cause we plan to book Silja for 1 way and Vikingfor the other to experience the difference)

In the past, I have booked based on price. The two lines both have loyalty programs but I doubt there would be much benefit for a tourist only taking two trips. If you are not staying overnight in Helsinki, there is a convenience to using Viking for both trips since the ship from Tallinn and the ship to Stockholm both use the same terminal; that would allow you to store your luggage in Viking’s terminal and pick it up before you depart.

We just did these exact routes last month. We did Silja and Tallink Baltic Queen. It looks to me all your questions were answered accurately. I did see spots where you could get luggage lockers at the Tallink terminal in Helsinki. Even going in March I was surprised how busy they both were so I would get your tickets ASAP.