This option is available for projects that you only plan on playing from a computer hard drive. In this case the project could be quite large and does not need to fit on a disc. There is no need to limit the size in this circumstance.
If you do plan on actually burning to a disc, it is not recommended to select the Unlimited size option. If you do select this option and your project is too large to fit on a disc, you will receive the error: “There is not enough space on the inserted disc to burn this content. Please insert a disc with higher capacity”. If you are burning to a disc, it is best to select the actual disc size that you are going to be creating.