Do floor trusses come in set lengths or can they be custom order

They are typically engineered and custom made for your application i.e. length and load rating. Truss companies will do the specs for you.

Hi ma, did you not finish your answer?

Edited the post, thanks.

Both, most lumber yards carry a good variety that will meet the needs of 95% of all applications. Many I have dealt with have you pick from their spec charts and then they putthem together for you.