You are right, HD Youtube videos come in many different resolutions and audio qualities. The HQ22 is the best (1280x720, stereo, MP4) but is not supported by the P3, you will need to convert it to smaller resolution. However, the HQ18 is good too (480x360 or 480x240, stereo, MP4) and you don't need to convert it. The P3 supports .avi and .mp4 but with some codec restrictions.
So, your FLV video downloader must show you the availables formats for downloading; if doesn`t, try some. The music video HQ22 is around 60mb and the HQ18, 20mb.