Accept, client-id and authorization should be headers.
Length should be sent in the body as a JSON:
curl -H 'Accept: application/vnd.twitchtv.v5+json' \
-H 'Client-ID: uo6dggojyb8d6soh92zknwmi5ej1q2' \
-H 'Authorization: OAuth cfabdegwdoklmawdzdo98xt2fo512y' \
-H 'Content-Type: application/json' \
-d '{"length": 30}' \
-X POST 'https://api.twitch.tv/kraken/channels/44322889/commercial'
You seem to have set everything as query string arguments.