What should be my VPS size.

This may be one of the most frequently asked questions online. The typical answer is an unhelpful "it depends"
The following is my experience with a 512 MB Single code VPS with Digital Ocean

After installing the stock 32bit ubuntu server OS my resource usage were

CPU 3%
Memory ~90MB

I then installed Perl, nginx, starman with 3 worker threads and dancer.
The CPU and and memory usage did not change.

I tried a load test with 100 simultaneous connection per second, the load test did not impact memory much but CPU went upto 100%
A similar load test with 1000 connections resulted in starman crashing

I finally installed postfix,mysql,dovecot,spamassasin

My current stats are
cpu 1.3%
Mem 212M/495M

So as long as I do not see very heavy load  a 512M VPS should suffice for my purpose.