I am looking for some solid explanation or better if there are any industry standards for choosing the server size as per the number of visitors and page-views? And, should one decide server size based on unique visitors or number of page views one gets in a month? Thanks
You need more information than just visitor counts. The only way to get the correct server size for your website is to load test it. Or buy a small server and see if it holds up, upgrade it if it doesn't.
Rather than look at hits per month, you should be looking at hits per second during your peak time. Your website needs to be able to stay up and serve content even during its busiest time. You should plan for a peak number of visitors that is at least double the observed peak.
Different websites have different performance characteristics. Some websites will be able to handle thousands of hits per second on minimal hardware. Other websites will need beefy hardware for far less traffic. There is a lot that goes into it:
- Which web server
- Which content management system
- Which database
- Disk speed and size
- RAM memory speed and size
- Network speed
- Web page size
- Whether pages are dynamic or static
- Number and types of resources per page
- If the site has computationally expensive actions such as resizing images
- Number of database hits per page
- If a CDN is used
- If a load balancer is used
Your website is unique. The only way to know for sure what server you need is to try them with a load test or live traffic.
If you are just starting out (and I suspect you are since you are asking this question), get cheap hosting. Rather than trying to plan for a lot of visitors, upgrade your server as you get them. Chances are your site won't be as popular as you are hoping anyway. Make sure you know how to set up a new server and migrate your site to it quickly. That is far better than paying a lot up front for a big server that doesn't get used.