It depends, honestly! They're big, I would say, but not huge? In a nutshell, Vocaloids are various voice-simulation programs, designed to be used as singing. (If you noticed how the voices in the videos don't sound completely human? They're not--there was a real person who provided voice samples, but it's a program who did the singing.) Each unique voice has a character design and name--if you've ever seen Miku around, she's one of them too.