What's the best length for a podcast?
Factually, it's determined by the listener. Too boring and they click onto the next on their play list. People like longer ones for exercising, shorter ones when they are at their desk.
Read the comments on the above link to see the real deal.
If you are using them for radio promotion, you should do some of many lengths. You need a practice run, or several, which are the length of a regular interview on the station(s) you are pitching to. Then you'll need a 3-5 minute version, which is for the short "Fox News analyst" type of plug-in. And you'll need a really short blurb (30 seconds or less) which is just a snazzy intro - showing how you can get your pitch out quickly.
Your media page should actually link directly to the MP3 file so that the interviewer can quickly listen/sample what your story is and how you sound.
The other podcasts, as I've mentioned, are for different publics - like to sell books.
Now, your media podcast should be in the form of an interview. Either get a friend to ask you the questions (as interestingly as possible) or read them off yourself ("...our next question comes from a listener in Puxatawny...").
The point is to get "on air" and develop your own "radio voice" and personality. Practice, practice, practice.
The great thing about podcasting is that you can simply blog your thoughts as they come to you - or write them out and polish, then read them like Earl Nightingale.
But save all your work, publish the best to your blog, and link the best of these to your media page.