What ad sizes can be used?
You can use:
728 x 90 - Leaderboard
468 x 60 - banner
234 x 60 - Half banner
Why only these?
Since the ads will only be shown below the Feed content you will have to be showing a ad banner which suits there, if you add a 300 x 250 then imagine how odd it looks.
How to add the ads in Feed?
Well you will need to follow the steps from my other post Add Your Own Ads For RSS Feed till step no. iii.
Go to Adsense account > My ads > Content > Ad units > + New ad unit > Create a 728 x 90 ad unit and click save and get code.
Copy the code and paste them in the place I've written "Code here" in the other image for the above suggested post.
This works 100% fine.