In Obby Creator you can only use a limited amount of blocks (parts). If you go over this limit, you will not be able to place or add any more parts to your obby. Any part can be sized freely and it will not count towards your limits any more than it did when you added it. Some individual part/block types have their own limits set.

Default limits per obby

Below are the default limits per obby that a new account has:

Overall Parts: 600
Buttons: 400
Character Models: 25
Conveyors: 200
Moving PartS: 200
Music Zones: 25 (Advanced Tools game-pass is still required)
Push Parts: 80
Quiz Parts: 50
Spin Parts: 200
Timed Parts: 200
Water: 50

Trivia

Part limits were introduced as a way for Obby Creator to make money but also to prevent lag, as many obbies with unlimited parts could make the game unplayable for everyone.