You can search TED talks by length -- and the 0-6 minute talks are about the word count (600-750 words) that you're asked to produce for your Task 1 presentations.
Each talk has an interactive transcript, so you can see how the talk looks on the page (and click the transcript to hear how it sounds when delivered). You can also check the word count for a given speech. Around 5 minutes is approximately right -- but much depends on the pace of the speaker.
Loads of subjects covered here -- watch several! Pick up tips and good ideas on how to present!