As others have said, it was just not practicable to involve the Soviets in an invasion of Japan even if the US had needed or wanted their assistance. There just wasn't enough shipping to relocate troops coming from Europe back to the US or directly to the Far East, to stockpile equipment for the invasion much less using any to move Soviet troops from Vladivostok to the invasion staging points in the Philippines or Okinawa. A Soviet invasion of Hokkaido wouldn't do much to the strategic balance. The Japanese knew where the main landings were going to be and reinforced those areas. A Soviet invasion of Hokkaido, if successful which is doubtful, would have only tied about 2 IJA division equivalents; not much in the scheme of things.
As far as size, Operation Olympic, the invasion of Kyushu, was going to be about twice the size of Operation Overlord. Olympic would have 9 divisions in the invasion first wave with another 5-6 divisions in immediate reserve and a reinforced division making the pre-invasion landings on offshore islands. Overlord had only 5 division in the first wave plus 2+ airborne division equivalents. The number of supporting warships, troopships, cargo ships and other logistics support was gigantic in comparison to Overlord, primarily due to the distances from the staging areas to the invasion location.