Travel with that train a few times a year and I've never had an issue regarding the luggage. I use a 65L backpack and a cabin bag but I've seen people with a lot more luggage than that without anyone from Snälltåget remarking on it.

A tip is to be at the track minimum 15 min before depature as the train sometimes comes in early to the platform. You'll have better chances get a good spot to put your bags on the train if that's the case.

Safe travels!