How long should a TOEIC Writing email response be?
No word count is published for the two email tasks; the target that matters is coverage.
ETS publishes no word target for the two email tasks. The only length guidance in the published Writing directions belongs to question 8, the opinion essay, where an effective response is described as typically containing a minimum of about 300 words. For questions 6 and 7 the target is coverage rather than length: six to eight sentences that address every required item in the directions, arranged in two short paragraphs.
A workable size in practice is roughly 90 to 130 words: long enough to answer the required items in separate sentences, short enough to write and check inside the ten minutes. The published criteria are the quality and variety of your sentences, vocabulary, and organization, so three clean sentences that each do a different job read better than one long sentence carrying everything. If you want something countable during rehearsal, count the required items covered and the grammar checks passed rather than the words.