I have a toy bunny which is about the size of a thumb, I don't have it with me right now and I want to let my friend imagine its size with my hands.
Is it correct to say "The toy bunny is about this big" or "The toy bunny is about this small"?
Normally you would say "big". You might say "It is small, about this big". You could say "it is about this small" if you wanted to be ironic about the size. You sometimes hear people say things like "I'm 40 years young" rather than "40 years old" for this reason.