I have created a level named Hundred. It is 100 x 100 cells and the solution is almost 200,000 moves. I designed it using Sokoban++ (it was the first engine I found that would support that large of a level). I designed it to be a test bed for development of large puzzles. While it is not the most difficult puzzle to solve, I think it does have merit. (What do I know? I am the author.
Would anyone here be interested in seeing/playing this level? (It takes a long time to play. I solved it in 4 sessions.)
If uploading it is an option, please tell me so I can do that. I have it saved in text format (the level followed by a level number). The solution is in a separate text file.