[YouTube-API] What is the channel image thumbnail size?

When I retrieve channel details from API, eg.

http://gdata.youtube.com/feeds/api/users/expertvillage?v=2

I get this in response:

<media:thumbnail url="http://i2.ytimg.com/i/QlGBspQdj17WOPBQMT1k9A/1.jpg?v=4f7b7aa3"/>

The thumbnail there has 88x88px.

If I take Rihanna, which has a default picture:

http://gdata.youtube.com/feeds/api/users/hollywoodrecords?v=2

I get his:

<media:thumbnail url="http://s.ytimg.com/yt/img/no_videos_140-vfl1fDI7-.png"/>

which has 140x108px.

Where can I find any specifications saying some limits about this picture (width x height)?

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