"How long should a cold DM be?"

Everyone says keep it under three sentences, attention spans are short. Myth.

Short isn't the goal — answerable is. A two-line DM that omits the deal, the deliverable, and the rate forces a creator to reply asking for all three, which most won't bother to do. You optimized for their reading time and cost them their answering time.

It's not length that kills replies, it's leaving the obvious questions open. Include the offer up front. Five clear sentences beat two cryptic ones.