At about 34:45 of the above video Donald Trump brags that he was at center stage of every debate, and goes on to say that he basically required it. At a speech in Waterbury, CT on Saturday he made the following statement about the debates:

“Every single debate I was on center stage, and the only thing I asked of the debates were I want an odd number of people. You know why? When it was an even number I was on center stage with somebody else. In other words, if we had five I was in the center, if we had six I was sharing it, I didn’t like that OK? So I’d fight, and I usually won that fight, but not always. But I was center stage, I was number one, on every single debate.”

Media Matters spoke with a representative at CNBC, who said that the placement was based on polling data alone, not any request by a candidate. Unnamed sources from CNN, CNBC, Fox News, Fox Business, CBS and ABC echo the statement that the candidates do not exercise control over debate placement.

Contemporary articles on CNN, linked from the Media Matters article, state that the December debate was determined by polling, while the February debate was determined by current delegate count.

Sources:
Video: https://www.youtube.com/watch?v=j2hpRTQRKdg
https://mediamatters.org/blog/2016/04/26/network-sources-deny-trump-claim-he-dictated-number-primary-debate-participants/210091
http://www.cnn.com/2015/12/13/politics/republican-debate-gop-stage/
http://www.cnn.com/2016/02/24/politics/republican-debate-houston-cnn-podium-seating