Is a huge hole behind a tonsil normal? it's like the size of a pinky nail

Could be normal: The tonsils have a surface that is not smooth. It has many crevasses called crypts. Sometimes these crypts can be quite a bit larger than you'd expect. However, if there are any associated symptoms such as pain, bleeding, enlargement, then that could be a sign of something more serious and in that case, the area should be examined.