I would like to introduce my nurse rostering solver "schedule-nurse".
Sudoku is NP-Complete
Nurse Scheduling Problem is NP-Hard.
Both can be solved by constraint programming.
You don't need to write program on how to solve, you only need to write constraint for the problem.
Nurse rostering software "schedule-nurse" is fast enough to solve huge blank sudoku, though it is not exclusive sudoku solver. It is free software, you can develop your own pencil game. From the point of another view,it may be the tool to get answer(SAT/UNSAT) for other fields of problem.It's just fun...
Is there a limit or maximum size to how big can be made?
That is the natural question. Crrent software can generate up to 36x36 sudoku. But the latest study shows 81x81 sudoku is possible within reasonable time. As for 100x100 sudoku, I can say it's very hard and challenging problem. To the best of my knowledge, I don't know such a sudoku generator.