We are finishing our basement and have a 54inch length of duct work we need to frame out. To give us the most head room we are laying the 2x4 flat. Does anyone know what lengh we need to keep the 2x4's (going across duct work)at so that when we attach drywall it doesn't bow?

This depends on the species of wood and the moisture content. Have you considered using steel 2x4s for your framing, much more rigid.

What is width of duct?

For best results, frame at 16'' on center. To increase your head room, u can use 1x2, 1x3 or 1x4.

Chuck