Instructions provided by UCAS say referees should write minimum of 1000 characters and miximum of 4000 characters for each applicant.
However, I cannot know what is the appropriate/standard number of characters referees should write for each reference.
Should write as long as possible within the limit without redundancy?
From your other post, you are the applicant, so you are not writing the reference (and to be clear, you only have one referee/reference). The advice you are citing is meant for the person who is writing your LoR, who UCAS presumes may be writing letters for more than one student.
The guidelines give referees an indication that the level of detail should be somewhere between a long paragraph and a page (4000 characters is give-or-take 500 words, which is give-or-take a page). But heavens above and sea below, NOBODY should ever just “write as long as possible”! If your referee has never written any sort of letter of reference, the part of the instructions they should pay attention to the “What to Include” and “Guidelines” section. It will be hard enough to fit that into the word count- same as you will find that staying inside the word count for your PS takes a lot of careful writing and editing.