Answer 2
Would you be:
Deoxyribonucleic acid (DNA)?
My name is akin to my size
The full answer, deoxyribunucleic acid is quite long, similar to the long strands that they are often found as. Meanwhile the initialism, DNA, is quite short/small and could refer to their small actual size
Each can be complex or concise
DNA can have many or few base pairs, depending on the genome
I remain within gaze
In front of your face
Observable under the right eyes
DNA is part of your body, therefore within gaze. To view DNA you would need a microscope.
My true prefix is one of a kind
My infixes are aplenty you'll find
Bit of a wild guess here... "deoxyribo" as the prefix would definitely not be common, while nucleic acids would be quite common I think this would be the suffix not the infix. So half answered for these two lines.
Even if one knew
What they attempt to construe
It would take them a very long time
It would certainly take a long time to create a DNA sequence from scratch
Considering freedom and span
I am not a favourable fan
DNA is often tightly wound in chromosomes, which would not grant it much freedom of movement. Its span is quite short as well.
Yet I still do support you
Will continue to guide you
Hold heavy loads as much as I can
DNA supports and guides our growth and development
In multiple forms I exist
In strenuous times I assist
With so much potential
I am quintessential
There are many forms of DNA, and sequences can be expressed when we are stressed. There's near infinite potential in what they can encode, and all living things have DNA.
Answer 1
In a similar vein to hexomino's answer, would you be a:
Microbiome
My name is akin to my size
Again, micro indicating the small size of the organisms that make up a microbiome
Each can be complex or concise
The composition of someone's microbiome can be complex (made up of many types of organisms) or concise (made up of a few types)
I remain within gaze
In front of your face
Observable under the right eyes
Since your microbiome includes what's on your skin, if you were to look at your hands for example, it would be in front of your face. However you could observe the microbes that make up your microbiome under a microscope
Even if one knew
What they attempt to construe
It would take them a very long time
Microbiomes are complex and it would take a lot of effort and time to identify each type of bacteria, fungus, virus, etc.
Considering freedom and span
I am not a favourable fan
Considering freedom - microbiomes are unique to each person, so it would be hard for it to transfer between people. Considering span - your microbiome is changing all the time, and the organism that make up your microbiome do not live for long
Yet I still do support you
Will continue to guide you
Hold heavy loads as much as I can
Our microbiomes can help prevent infection, help digest food, i.e. supporting us. Holding heavy loads might refer to dealing with microbial loads, where our microbiomes keep heavy loads (large amounts) of harmful bacteria at bay
In multiple forms I exist
In strenuous times I assist
With so much potential
I am quintessential
Once again, referring to the fact that the composition of microbiomes varies and that they benefit us. Quintessential in that everybody has one in some form