What is the ruling on wearing a Swiss watch that bears a plus sign (+) as a trademark, knowing that the lengths of the lines of the plus sign are equal, and is it considered similar to the cross?

One should not be expansive in doubts to the extent of imagining that every two intersecting lines are the symbol of the cross. There is no harm in wearing this watch or similar items unless one has certain knowledge or a strong presumption that the company intended the symbol of the cross.