50 Watt Stirling generator

[ontuyalex]

Hey i'm working on a project. I am building an automatic greenhouse
that manages waterings and temperature and whatnot. I've been
designing and redesigning it and I have the total power use down to
under 50 watts (with surges during heating times).

My question is, How do I size a Stirling engine to generate 50 watts
of power. I am kinda in the dark when it comes to the conversions
and whatnot of Stirling desing. Also how big would the thing have to
be?

Thank you for answering