I had a 20 by 40 and it seemed a little long. Surely would take too long if training several dogs a day on it. Filled it in and now I have about a 15 by 30ish that works well. Might make a little longer sometime but it works well for now.
The swim by pond at Nan-Dool Kennels is about 45' by 129'. It wasn't designed tobe that long, but by the time we removed the green ash stump on the end it became 30' feet longer. When I tried to pull the stump it almost pulled the dragline over, but the stump yielded. That is why some areas are irregular.
Below is a picture digging before the swimby was dug,about 40' behind the dragline.