In my experience, even if the center speaker is very important, you cannot decide on the center without considering all the other speakers. At the minimum, you can’t really audition a center speaker without listening to the fronts at the same time. Those 3 speakers act in unison.
I don’t want to suggest a particular speaker because I don’t know your total budget but I’d say that one of your biggest concerns will be power. The room is huge (72 square meters) so you will want to get highly efficient speakers. Probably 92 db or higher sensitivity.
Related to this, given the size of your room, you almost surely need more than a 5.1 system. I would suggest a 9.1 system with Audyssey DSX Wide and Height speakers (which the Onkyo 5008 supports). You can forego the Surround Back speakers.