What is the capacity needed in my icloud to purchase Downton Abey season 1&2

Your iCloud account doesn't store your purchased TV Shows. They are available to redownload from iCloud for free in some countries (see http://support.apple.com/kb/HT5085), but they are not stored in your account. When you purchase them you store them in your iTunes library on your computer. You can then sync the episodes you want to have on iPad using iTunes.

If iCloud doesn't store purchases, what does it do? In the library it indicates that I should download the music, program, whatever from iCloud.

iCloud is primarily used to sync "Apple data" between devices and to store specific data files and hold backups of iOS devices. Apple data refers to contacts, calendars, email, reminders, etc. It doesn't store your purchases within your account, i.e. purchases don't count against your storage limit.

iCloud is designed to keep your data in sync across your devices, to share phptos and to back up your device. It will also stream your music library if you subscribe to iTunes Match. It will also allow you to renown load previously purchased music, apps, books, tv shows and movies is you should lose them, assuming they are still available in the store. It doesn't, however, store your purchased media; it simply allows you to redownload it it it's still available. You should be storing your purchased media in your iTunes library.

Thanks. Question answered. Now I understand.

You're welcome.