Though I have asked a similar question here, and I am very thankful for NautArch's answer, I realized that the more I looked into the text of a changeling's shapeshifter trait, the more questions I had as I am not quite understanding their limitations and specifications.
To reiterate the text:
As an action, you can change your appearance and your voice. You
determine the specifics of the changes, including your coloration,
hair length, and sex. You can also adjust your height and weight, but
not so much that your size changes. You can make yourself appear as a
member of another race, though none of your game statistics change.
You can’t duplicate the appearance of a creature you’ve never seen,
and you must adopt a form that has the same basic arrangement of limbs
that you have. Your clothing and equipment aren’t changed by this
trait.
The first part of the confusion I have is knowing what exactly you can change into. Adjusting height, weight, color, (gender, hair length), and race so long as they are a medium creature sounds simple enough, but does this mean you can be any creature with 4 limbs in a medium-size? From a medium minotaur to a wolf? From a Babau to an animated armor? What about see-through creatures like Shadows?
The second part of the confusion is understanding the nuances of how sizing works. While I realize that the basic rules focus on player space, I found that the System Reference Document of 3.5e seemed to comply with 5e's guide when checking playable race height guides, so good as a guide to find out how tall/short I can make my changeling.
According to that, a changeling can be anywhere from a 4ft 60lbs character to one of 8ft and 500lbs. However, as a changeling's stats don't change and things like speed and lifting strength are not tied into the size of a creature, does this mean that your changeling monk with the mobile feat and a +1 strength (for instance) could appear to be a 4' dwarf weighing 60lbs carrying 225lbs at 60ft per round the same as an 8' tall goliath with 500lbs of pure muscle unable to do any better?
I realize that these examples might seem exaggerated, or quite different, but there are there to highlight the confusion that I still have with the changeling's feature; the monster type is described in 3 vague ways (descriptors, race, creature) leaving little understanding to the limitations, the stats remaining the same seem odd if one can gain or lose muscle mass, and I can't seem to wrap my head around it.
Last note, I realize this question is quite similar, but I am still confused and was hoping for a bit of clarification.
Best Answer
Changelings can impersonate medium sized people they've seen
You can imitate another medium humanoid creature including a dwarf or goliath
Both Dwarves and Golaiths are medium, so both are fair game for the changeling(Eberron: Rising from the Last War p.18):
The limitation of the extant of the alteration is such that it would not change your size category.
Changelings can imitate different races, but not different creatures.
The changeling trait(ERiftLW p.18) limits them to another race, but not another creature type entirely:
This also eliminates wolves, animated armor, skeletons, etc...
Race vs Creature Type
Opening sentence of choose a race section of Player's Handbook (p.11) states races are species of the humanoid creature type:
Races encompass what the world considers people and have the humanoid creature type.
Race in the monster manual is provided as a tag of creature type. See soldier or archmage for an explicit example of this tag being listed as race:
Other examples of race tags are: lizardfolk, sea-elf, halfling-musician, warforged-soldier
A spell effect that has a race changing effect is reincarnate.
Only races with same limb arrangement.
Some races are not illimitable as they don't have the same arrangement of limbs such as centaurs or aarakocra (depending on how you count wings). The changeling trait limits the changes to those that match their own limbs:
Equipment
The disguise is more than just adapting the form of another person. The equipment is not included in the transformation.
Thus, a diminutive dwarf, while still being medium, might attract some attention while carrying 265 lbs of equipment. Similarly, a muscle bound goliath struggling to lift a load would likely draw unwanted attention. These are considerations changelings must account for when imitating a person.