<p>I'd go for the more concise the better, to increase the chance people actually read the tag wiki excerpt before using the tag &amp; perhaps therefore use it correctly.</p>

<blockquote>
  <p>The usage guidance, or tag wiki excerpt, is a short blurb that
  describes when and why a tag should be used on this site specifically.</p>
</blockquote>

<p>Mousing over a tag &amp; reading the excerpt involves deciphering tiny white text on a grey background while simultaneously maintaining the cursor positioned over a small grey box. This aren't circumstances conducive to learning what distance covariance is; but to quickly checking what the <a href="https://stats.stackexchange.com/questions/tagged/distance-covariance" class="post-tag" title="show questions tagged &#39;distance-covariance&#39;" rel="tag">distance-covariance</a> tag is for, &amp; what it's not for: I, for one, am grateful for brevity. When adding a tag to a question, you're presented with a box containing six tag wiki excerpts, which is updated as you type: concise excerpts aid a speedy choice of the correct tag. If 460 characters is a wise maximum bound it doesn't follow that approaching 460 is an optimum.</p>

<p>I don't think the full wiki is buried very deep&mdash;it's one click away from the excerpt pop-up&mdash;, &amp; it's the right place to read, in comfort, a fuller explanation of what the tag refers to.</p>
