Here's a patch. The default fragsize chosen is 70 as that is the same used when the regex fragmenter (of the original Highlighter) is used in Solr. These are both similar in that you typically want to shoot for a passage about a sentence in length.
Note the regex fragmenter has a "slop" feature that is 60% of the fragsize... this is not (yet) supported by the UH's LengthGoalBreakIterator.
LUCENE-7620 lands (this weekend?), I plan to commit this immediately after.