It sound like you'll be knitting back and forth on the circ, not in the round for a table runner. In that case the length of the needle won't really matter.
I'd probably go for something in the neighborhood on 25 or 26 inches tip to tip. The "standard" sizes the needle makers use are 29", 24", & 16". I've occasionally seen 32" (in really large sizes like 15, 17) and I have one OLD circ which is 20".
I tend to like 24" myself - it's long enough to put a sweater body on without being too long, even if you're doing waist shaping ( for my very thin nieces sometimes 29" gets too tight) but there's still enough room to get your hands on the needles without feeling cramped.
HAPPY KNITTING! :XX: :XX: