Willeke
I guess you will travel with either the French or the Belgium railways and neither has restrictions on the size of your luggage.
But it might be a bit of a struggle as you will have to take care of your own luggage (no help, no check in luggage,) and not all trains have a lot of luggage space.
No extra payment for the luggage needed.
Rail Europe is just a ticket seller, not a rail company. I would buy my tickets from the SNCF, the French rail company, as they do sell at the lowest price and you can pick up the tickets at your start station.