>>How high can a cobra stand up? Is it 1/3 its body length?
A good general rule of thumb is that most cobras do stand 1/3 of their body length in height. However, like any rule of thumb, that can be incorrect at times. Some of the other "cobra-like" animals, like rinkhals spitter, tend to stand about 1/2 of their body length. Most of the pictures I've seen of them have been like this, and the young female I have does this as well.
>>Also, can a cobra stike only as far as it raises up?
No. Although a fair amount of people think this, I've seen cobras strike from a hooded position and be so adamant about wanting to bite something their body actually shot forward more than it "should" have. So your best bet is to never count on this "fact" to avoid a bite.
>>Example: Can an 18foot king cobra raise off the ground about 6ft.
Theoretically yes, but then again there haven't been many 18' kings around (only one I believe, and that was the record) to test out the idea. I would be inclined to believe, however, that an 18' king could most certainly stand up tall enough to kiss the average adult male human being right on the lips.
>>And if that cobra raises off the ground 6ft, it can only strike a distacne of 6feet, right?
Again, you're getting on shaky ground here. With matters of live an death involving highly venomous snakes, it's generally a good idea to not use the words "only strike a distance of such and such feet," etc. I've seen cobras strike so hard they propelled themselves, strike will shooting forward (forest cobra style of attack), strike from a non-hooded position, and many other ways. Anyway, I hope my banter helped.
-Chance