I am accustomed to seeing 320x240 size, and I don't think the clips should be shorter than 10 seconds. I think 10-15 seconds is fine.

Split the files into different focus clips. Don't make 4 clips that add up to one 40 second clip in sequence... jump ahead from lets say kissing to blowjob to assplay to fucking... as an example... without giving away too much and will get the surfer off, but it lets the surfer know what content to expect from the site.