A PhD LOR must speak to research capability specifically: independence, persistence through failed approaches, and the quality of the candidate's own questions. Teaching performance is secondary at this level.
Someone who supervised your work directly and can give specific examples. A junior lecturer who taught you closely writes a far stronger letter than a department head who barely knows you.
One page, three to four paragraphs. Longer letters dilute the specific evidence that makes them persuasive.
It happens, and some recommenders ask for a draft. The recommender must review, edit and sign it in their own voice — the letter has to be genuinely theirs.