String theory: the Planck length
In string theory, the scale length of strings is taken to be the Planck length, based on a few fundamental constants by Planck. Planck noticed that it was a small size, 10^-35 m. Everyone since then seems to take the Planck size, which is as arbitrary as sizes come, as the smallest size in physical theory. However, the black hole radius of an electron mass is only 2GM/c^2 or 6.8 x 10^-58 m, much smaller than the Planck size.
Notice that string theory since the 1960s has relied on strings all having the same fixed size of about 10^-35 m. This means that the string tension would have to vary to allow different fundamental particles to be strings of equal scale length. However, if the strings are rolled up into loops of a black hole size, the radius increases directly with the mass. This makes me wonder whether the reliance on Planck dimensions makes sense.
As pointed out in previous posts, there are arguments from the Standard Model for 10/11 dimensions.
The fifth dimension can represent the spacetime fabric which appears in 4-D spacetime as the gauge boson the graviton, the light speed radiation which causes gravity. Individual units of string theory, like Witten's M-theory, are self-consistent, but the whole of string theory includes many different and contradicting ideas about branes imbedded with particles which give lie to the concept of a self-consistent string theory.
A brane on a 5-D spacetime is 4-D (just as a 2-D membrane exists on a 3-D bubble).
A 4-D spacetime is a hologram of 5-D (just as a 2-D hologram contains 3-D information).
We already know in general relativity that 4-D spacetime can be used to model gravity as causing a curvature in 3-D space. General relativity is often 'explained' as the curvature of a rubber sheet causing indentation so that balls placed on roll towards one another. This is manifestly more obscure than need be, because the real space fabric is not two but three dimensional. Some of these abstract equivalences are actually a hindrance, not a help.