The PDGA Course Directory is not yet set up to collect pars and lengths for each different tee-to-pin combination. That will be a major Course Directory upgrade effort needed to support enhanced app capabilities and live scoring from multiple groups. Planning is underway for that.
In the mean time, the Directory provides room for two course lengths entered by the person who updates the Directory listing. These two course lengths might be useful if there's only one pin and two tees per hole. However, with multiple pins on many courses, the current pin mix in play is likely a course length in-between the two. Unfortunately for now, the app user will need to know something about the course and estimate its current length using the two lengths in the Directory for guidance.
There's no advantage in selecting your course length in advance of the round until individual hole data is available in the Directory and can be uploaded to the app. For now, entering your course length at the end of the round is as good as at the beginning in terms of doing the round ratings calculation.
Rater of the tossed arc.