Air Carrier pilot reported a lack of taxiway markings at DCA causes '...a safety hazard for both clearance issues with taxiing aircraft; but also in keeping your wheels out of the dirt.'

Narrative

Taxiing to Runway 19 from Gate XX; we were given taxi via K hold short of Runway 15; which we did. Then we were given a clearance to cross Runway 15 into the hold box; hold short of Runway 19; you're #3 for takeoff. As we entered the hold box; we were told to pull up 90 out to the river; short of Runway 19. We did all that was instructed. I have a huge concern over taxiing into the hold box; as there are no taxi lines and no indication on where to pull up to; so you do not impede aircraft taxiing for takeoff on Taxiway Juliet. Without a taxi line to show you how close you are to the edge of the taxiway; and no indication on how far to pull up short of the taxiway (only the runway hold short); it becomes very difficult to tell if you are clear of aircraft taxiing for departure; and that they can clear past your aircraft.This to me appears to be a safety hazard for both clearance issues with taxiing aircraft; but also in keeping your wheels out of the dirt. We had no incursion; or close call; but I felt other aircraft had to deviate from the centerline to feel comfortable passing by us trying to get to the runway. I really believe there needs to be some taxiway and hold lines in this hold bay if we are going to continue to use it. We had no incursions or runway encroachment issues; This is a report to identify a possible future safety hazard. Suggestion. Taxi lines in the hold box; as well as a position to hold that will ensure your entire aircraft is clear of all runways / taxi ways and other aircraft.