A Back Bay home may now make the claim to be the biggest home in all of Boston. Investment executive Ofer Nemirovsky bought out four condominums in his and an adjacent building to merge them and create a 24,000 square-foot mansion. I guess he needed the space. The home, when completed after a two-year renovation, will feature 15 bathrooms and six parking spaces. Six! There are people who would give an arm for one.