I've helped a lot of people buy laptops over the years based on description alone (I eventually wrote a wiki page to help people decide on factors). Mostly it boils down to portability - after I explain what it's like carrying around a 15" item, most (not all) start looking at 13" or smaller. Being guided on screen size is the sole attribute that people mention after the fact (usually 'thank you' or 'I wish I had listened'). The rest is much of a muchness and varies on more detail in the use case.

Sometimes a 15" or bigger is warranted, but I've only seen it suitable for the rarely-mobile, not the always-mobile.