Here is a useful resource (with very good guidelines) about how to calculate the required transformer VA rating. Here is the link:
http://www.atc-frost.com/products/design/va.htm
And some notions about box design, temperatures... :
http://www.atc-frost.com/products/design/
http://www.atc-frost.com/products/design/va.htm
And some notions about box design, temperatures... :
http://www.atc-frost.com/products/design/