As described in several Jira requests, including this one:
https://jira.secondlife.com/browse/VWR-1094
When this issue is requested in the Jira, Linden responses have typically been, "SL is a virtual world, there is no need for people to know their height." This is very disheartening. What is being requested here is a visual design tool, for a platform that relies heavily on visual design elements. Denying such basic design information to all residents has caused so much damage to SL, and is one of several root issues for SL's well deserved reputation for poor visuals.
Nearly all aspects of content creation require a basic understanding of height and scale. These elements are some of the earliest basics taught to those seeking a professional education in visual design fields.
Animations will not work for avatars of all sizes. Almost every resident has probably experienced an animation which has caused them to float above the ground, or to sink below it. A few animators have caught on to this, and now provide animations tailored to specific heights, but these are the exceptions. Most animators, and the vast majority of residents, remain completely unaware of the issue, even when they experience the problems it creates.
Avatar attachments must be sized for avatars, as most give no thought to scale and design around their own avatar it has made it difficult for those whose avatars vary too greatly from the avatar of any given attachment's creator, especially if the purchasing avatar is shorter.
Scale is lacking entirely from environment design within SL. Again, most people build around their own avatars rather than a universally defined sense of scale. Furniture, especially no-mod furniture, is sold without any information with regards to what size one's avatar must be to properly use it. A properly educated user base would be more prone to designing such items with scale in mind, and releasing products in multiple versions to suit avatars of different sizes.
Many have long requested an expanded height range, not realizing that it is already possible to create nearly 9 foot tall avatars which would loom tall over human sized avatars. Unfortunately, as the average user already pushes the tall end of the spectrum (SL's default avatars are just over 2 metres tall! Many push their height further, still.) it is impossible for one to create an avatar taller than "average".
Most people do not intentionally create avatars over 2 metres tall. They do so because they are not given any indication as to what their height actually is. There is a significant number of residents who do attempt to design an avatar to a specific height of their own choosing, however most of the resident created "height detectors" do not provide correct height. This confusion has caused even more problems in creating content.
The lack of height information has also damaged SL's ability to provide immersion and engagement. It is impossible to achieve a high level of either in such an environment, for the obvious reasons. Much like poor proportions, even if one cannot quite put their finger on what is wrong or how to fix it, most will realize something is wrong. For SL, this often manifests as broad criticism regarding SL's visuals.
From a resident's viewpoint, it would seem like a simple thing to add. A small line of text within the avatar editing window which displays your current height. It is only reasonable to provide this information to users, especially as SL relies to heavily on user generated content.