Q:

Estimate minimum hardware requirement of a webservice at design time

is it possible to estimate to minimum hardware requirement of a web service at design time?
i.e. based on the estimation of input size, response time and etc.

A:

IMO, you will have to do some analysis on the expected request/second, and how many requests do you want to be able to handle? 
Also, you can scale and distribute a web service easily. So a little skew wouldnt be a problem.
without knowing your application, it s very hard guess what kind of server you will need. A blade server with xeon processor easily handles about 2K requests per second. I did work on a application where we were able to process 3K requests per second on a blade server. Note: we had minimal data access, we were using in memory cache and distributed cache. 
So there are lot of factors to consider while you are doing capacity planning. That s why you can start with small hardware and then you can scale your application/hardware horizantally or vertically based on your needs.

