Question
I would like to buy a deciduous tree for screening my neighbors house in CT. What is the most economical size to buy a large tree and which kind grows rapidly?

Answer
Without knowing the space and site you are going to plant this tree in it will be difficult to recommend a species. I would suggest that you contact the CT department of Forestry , Urban Forest Coordinator, Chris Donnelly and he will be able to give you local advice on which tree would be best for you. Here is the web link to the contact page of the Department of Forestry.
http://www.ct.gov/dep/cwp/view.asp?A=2697%26Q=322872#learn