How to express "I think" (not I feel) I'm wanting to say, "I think China is the world's second biggest country." As, in, "I'm pretty sure this is the fact."  

I would have originally used 觉得. But apparently this is only for opinions? As in "I think your Chinese is good."

I'm not wanting to say, "I feel" or "I believe", i'm wanting to express that regarding a certain question, I believe the FACTUAL answer is such.

Thanks very much.
"我认为" is good, it's equal to "I think", which means you deducted it from your knowledge.

You can also use "据我所知" witch equals to "As I know".

"我认为中国是世界第二大的国家", or "据我所知，中国是世界第二大的国家".