There are WAY too many design factors to easily answer your question. I would recommend finding the following books at your engineering library, I read these last year while in college and they helped me understand engines and all the factors that go into designing them much better. The first is "The Phillips Stirling Engine." This book got me so pumped about Stirling engines it is ridiculous, it has a wealth of information. Another useful one is "Stirling Engine Design Manual" by William R. Martini, it even has some Fortran code for a 2nd order model to predict engine speed and output etc(I've never used it). The books written by James Senft are also outstanding. I liked "The Ringbom Engine," as well as his other ones on model engines. There is one book I haven't read called "The Stirling Engine Manual" that looks like it is good, but it is more geared toward model engines. These books should help out a great deal...also the one on the 5HP engine on this website looks like it would be an excellent resource. Good luck with your design, I hope you can find these books because they are a great help.