According to the official blog of Ichiko Hashimoto
, Code Geass Gaiden: Boukoku no Akito
will be a four episode film & OVA series, similar to the format for Mobile Suit Gundam Unicorn. Each episode is about 50 minutes long and will be premiered every three months followed by the Blu-ray release. The first episode will be released early this summer.
Code Geass Akito official website
Trailer on Youtube
Trailer on AnimeOne
(might be IP-restricted)
Source: Hashimoto's blog