There is no real limit as to how large a vehicle could be and still move, assuming that you actually want as big of a vehicle as possible.
Armor of a given thickness actually makes up a smaller percentage of a vehicles weight the larger that vehicle is, because the armor weight only increases by surface area, while the total weight increases by volume. Armor weight increases by the power of 2, other weight by the power of 3. This means that larger vehicles can have thicker armor, or can spend that saved weight on other needs.
Allowing the vehicle to move is relatively simple; just keep adding engines until it can. Divide the tracks into segments and have each engine work a single segment. Sinking into soft soil isn't much concern either, just make the tracks wider. The ground pressure of an M1 Abrams tank is only twice that of a person. If you wouldn't sink when standing on one foot, neither would the Abrams. Crossing rivers isn't either, assuming your tank is truly gargantuan. Who cares about finding a strong enough bridge, just ford the river. But there's a problem. Tanks don't generally drive strategically-relevant distances under their own power. How are you getting your super-tank on a train? What about a ship? A tank this big would be a nightmare to repair, meaning you only want it running (and suffering wear and tear) in battle. Not driving 500 miles to battle.
As others have said, tanks much bigger than real ones are easy targets for airstrikes. Especially before the modern day. Some nations have tested active defenses on tanks to protect against ATGM's, but these are still rare and not super effective. If you can provide your super-tank with some kind of point-defense system to shoot down incoming bombs or ATGM's, it could deal with air attack.