I am remodeling an old farmhouse. The ceiling joists are 26" on center. I would like to drywall the ceiling, but am afraid the span is too great and will cause sagging between the joists. Is 26" too large a span for 3/8" or 1/2" drywall? Would furring strips on 16" centers perpendicular to joists give enough support? Any ideas?

When ceilings are framed on 24-inch centers, it's normal to use 5/8-inch drywall. The extra 2 inches you have between joists shouldn't matter. You will find that 5/8 drywall is quite heavy, so have some robust help on hand when you apply it.