<small> is in the ‘4.6 Text-level semantics’ section of the WHAT-WG HTML living standard. <small> one used to mean just that: small font. It is now one of a set of elements that has been re-imagined in a more semantic light. <small> now has a very distinct meaning – it now represents ‘side comments such as small print’. This also would cover disclaimers, copyright notices and the like.

<small> is a text level element, so is expected to be used within a block of text such as a paragraph, and not as a block of text. So small print such as ‘plus VAT’ added after a price would be appropriate, whereas a long body of terms and conditions would not.

An example, first as rendered and secondly the HTML itself:

this was my interpretation of the small element All details believed to be correct at time of writing, please note the HTML5 spec may change without warning

this was my interpretation of the small element <small>All 
details believed to be correct at time of writing, please note 
the HTML5 spec may change without warning</small>

I’ve had a look around a sample of sites, and couldn’t find any examples of this being used in the wild unfortunately. As such I think for now I’ll consider this in the ‘making presentation elements have a semantic meaning’ mop up, use it as and when it makes sense. If anything it serves a good purpose as an illustration of the shift of thinking needed for some developers that HTML should be semantic and not contain presentational information.