I believe you can go as high as 255/60R15 on the rear and 235/65R15 in the front due to the tire clearance when you turn your wheels make corners.
This will keep the diameters of your front wheels and rear wheels the same, but the front wheels will not be as wide so you can turn your wheels without it hitting the fenders during sharp turns.
Good luck and hope this helps answer your questions and i believe you will not have any speedometer recalibration needed with this size tires. There diameters the same as your stock tires. 205/70R15