Well shiver me quivers!
So, we just need flavor text and weight?
Yup.
Arguably we don't need the weight, since the SRD Snakes don't have it.
Anyhow, a typical King Cobra is about 3 to 4 metres long and weighs ~6 kilos (or 10-13 feet and ~13 pounds). The largest King Cobras grow up to 18 feet, but rarely exceed 45 pounds in weight.
Here's a King Cobra the text says is 12 foot 3 inches long and weighs 16 kilograms. However, I suspect it might be 16
pounds, since (a) the length is in UK/US measurements and (b).
So, if we assume 12 feet and 15 pounds for biggish King Cobra, then an 18 foot Giant Cobra would weigh 50 pounds if it has the same proportions.
A 12'3", 16 kg cobra would weigh 111 lbs if scaled up to 18 feet.
Hmm, both of those weights seem mighty low for a Large animal.
Still, king cobras are slim snakes. If it had a thick body like a diamondback rattlesnake it would weigh somewhere around 400 pounds when upscaled to 18 feet long.
Alternatively, we could increase the average length to 24 feet for an average Giant Cobra. That's a better match to the lengths of a standard snake/worm shaped creature (e.g. Purple Worm is 80 ft. & Gargantuan => 20 ft. & Large).
I think I prefer the "make it longer" solution. How about:
Giant cobras are usually between 12 and 30 feet long, but they can grow considerable bigger than that. A typical specimen is 24 feet from nose to tail and weighs 500 pounds.