Ruby Sinatra and PerlDancer are called "Micro web framework"?
What does it mean to be a micro framework?
Are micro web frameworks suitable for large projects (say Stackoverflow, e-shopping, real estate management or ...)?
What are the best usage cases for micro web frameworks?
There isn't a perfect dictionary definition for "micro framework", but the python micro framework, Flask has a decent explanation here: what does micro mean.
As to whether or not it would be appropriate for a complex website, experience will guide you. As your project grows in complexity, you may find that you outgrow the micro framework, and may prefer something that promotes better code organization in a large project. But if this is a side project, don't let the fear of outgrowing it prevent you from starting with a micro framework that you're comfortable with.