If a naga was 7' tall how long would it be?
Let me establish two points first that the previous answers sort of miss.
When a cobra looms up to be at eye level with a human it is reaching its body up higher than it does normally to be able to do so.
As is illustrated in the picture with the bracket, when the snake rears itself up just enough to be in an upwards posture the bracket notes that this part of the snake would be the "standing" part of the Naga and would be 7 feet tall.
With these two details, the proper height and length of a naga can be determined.
In essence you are combining two creatures together, so where and how would they do that? The upper torso and lower torso of the average human male are usually about the same length, though in most situations the lower torso area is slightly the longer of the two parts. This is usually only by a few inches though. But what this means is that of the part of the snake that is "standing" in the picture, a little less than half of that will be the upper human torso of the naga.
If the naga in its "standing" position will be reared up at about 7 feet, than nearly 3 and 1/2 ft will be the upper human torso. With that being said, we now have a solid proportion for how big the rest of the snake would need to be. Now that snake looks to be rather small, not nearly the full 14 feet its said to be able to be at full size. With an understanding of how big the "human" torso section of the snake would be, if we could find the actual size of that snake in inches, then we could gain an understanding of the naga's proper size.
Basically, lets say that the marked section of the snake is actually 7 inches, that would mean that we would have a 1 to 1 ratio: for every 1 inch the snake is, the naga would need to be 1 foot.
This is because we're trying to find the scale of the "standing" portion of the snake in regards to when its enlarged to be a naga. If that portion is 7 inches, and that section is supposed to represent the height of a 7 foot human male, then each inch would scale to be one foot.
Now for a bit of an anatomical side note
Even though the naga wouldn't have legs like a normal human, it would still be able to rear up and "stand" as normal human would do. Whether it would be able to move like this and slither is up to whether snakes can actually slither and move when they are reared up as well, or simply the creative liberties of the person creating the naga. BUT, and this is a big but, the lower (aprox.) 3 and 1/2 ft of the naga would still in essence act as its "feet" or legs when it comes to the proportional act of standing up
End side note
So what this would mean in total is that if the average cobra can become 18 feet at its full length, then the full naga would be able to pulls out calculator
...Oh wait, it seems I've forgotten to mention something. The example of the snake being 7 inches at "standing" was completely anecdotal. To actually calculate the height and length of a naga you would first have to find out how "tall" the standing portion of the real life snake is in INCHES, based off its individual species and current age,
Another anatomical side note
As a cobra gets older it also gets longer, and the "height" of its standing position will scale up. But would a naga's "standing" height also scale up, causing its upper human torso to eventually become much bigger than that of a normal human's, or would its upper torso stay the same relative size as that of a human's? Because if the upper torso does not scale with its snake body and become proportionally bigger, but rather stays the same relative size as the human body, then that means there is a bit of anatomical discrepancy between the way a snake and naga scale, and that it may not be as much of a 1 to 1 ratio as previously stated.
End side note
This is because half of the snake's "standing" height would be converted to the (aprox.) 3 and 1/2 ft of the upper torso of the human body. So if there was a snake that was 1 ft tall "standing" then every six inches of the snake would be (aprox.) 3 and 1/2 feet, or every 12 inches would need to be 7 ft. Meaning that a snake that was 1 ft "standing" and 18 feet long would become (7[ft] x 12[in]) 126 feet long as a naga.
And as a final anatomical note, as I previously mentioned, when a snake looms up at a humans eye level they are purposefully extending themselves up much higher than they usually do. When scaled to a naga, if a snake can loom up to a humans eye level at -let's say- 6 feet, and their total body length was 18 feet, then you would have to proportionally increase that for the naga accordingly. If the snake was 1 ft "standing" and could loom up to six feet, this being about 1/3 of its total length at 18 ft total, that would then mean that a naga could, at 7 ft tall, and 126 ft long, loom up to (aprox.) 42 ft. Which would be nearly twice as tall as a Storm Giant in the current 5e universe of dnd.
As a final note, I want to emphasize this scale only works if you are trying to scale Nagas based off real world snakes and proportions. This scale does, though, provide a good reference for how long a naga would have to be,
((([Length of actual snake in feet] x [12 inches]) / [Height of "standing" portion of actual snake]) x [Intended "height' of standing portion of Naga])
Snake 8in "standing" portion
18ft long total body
18 x 12 = 216
216 / 8 = 27
27 x 7 = 189 feet total