Generally, this distinction depends on symptomology.

The small fibers are the thin, lightly myelinated or unmyelinated fibers that subsume the sensations of pain and temperature, and also underlie most autonomic functioning. So one who has burning or lancating pain, numbness and or disrupted temperature sensation generally has some small-fiber component. Depending on the specificity of the symptoms, one may even be able to narrow down just which types of fibers are being attacked (although the attack can be global, too).

Larger, myelinated fibers include all motor nerve fibers, as well as those that underlie the sensations of vibration, mechanical touch, and bodily positional feedback. So disruption here presumes some problems with those fibers.

People who have global autoimmunities, or whose neuropathy includes the nerve roots near the spine and/or the ganglia, typically have both types of fibers involved.

Only large fibers can develop demyelination, though, as the smallest fibers do not have myelin sheathing. Small-fiber neuropathy therefore is by definition axonal--the damage is to the fiber itself. Large fibers can be damaged both by demyelination and by direct attack on the axon, and often both occur simultaneously, or one is secondary to the other (i.e., demyelination may expose the axon to assault, so one has "demyelinating neuropathy with secondary axonal degeneration").