Here are 3 steps to identify your market and calculate market size
1- Target Market
Finding a target market, helps in spending marketing and promotion money efficiently on your ideal customers.
A) Focus on the market
- Understand the problem that your product solves
- Find out, what needs your product or service fulfills 
- Funnel approach: Use a funnel approach to filter the data according to gender, age group, area, income etc.
- Value proposition: Find out what type of customers will value your product. Example- if you manufacture a hiking bag, then hikers will be your target audience.
B) Market Research Data
- Survey Data: Do survey via email, newsletter, phone, in person etc. Find the data and analyze it for your product.
- Existing Data: Find out if there is any product similar to your product.If there is a similar product then analyze its market and customer segment. Find out the market segment of that product. You can go different coffee shops or bookstores to talk to people and get views of the people for that product. Start visiting review websites and find out negative reviews for the product. Write down the problems which people are going through example- bad customer support, bad service etc.
- Self-Market Research: Meet your family members, friends and colleagues. Tell them about the available product and its features. Ask them to try it or look at it directly or online. Get their feedback. Note down the negative feedback and now tell them about your product and its features and ask for their feedback. Analyze their feedback.
The analysis of data coming from different paths can help you to find out your target customers and increase return on investment.
2- Competitive Analysis
Find out your top competitors and analyze their value propositions. Knowing about your competitor’s value proposition will help you to find opportunities and threats for your product.
You can do the SWOT (Strength, Weakness, Opportunities and Threats) analysis of your product. Having a very different value proposition will make your product stand at the top level with least competition.
3- Calculate Market Size
Especially when it comes to decision making, market sizing is a very important factor for your business success. It helps you to find the market trend and required investment for your business
A) Define your target customer: From step 1, you will be able find target market features and who is your target customer. Now create personas of your customized market.
To create a persona, please read my article “ 9 Steps to Make a Simple Marketing Plan”
Case study: (Assuming) We are developing a healthcare software for patients who can get their medical reports via an app. They can upload their diagnostics report or ask a question to doctor through this app. The primary target customers are 66% of U.S population who are using health apps. 
B) Calculate number of target customer: Find out the total target customer number, which may have similar profile like the targeted customers.
Case study: By doing a general market research, we found that the target customers are 213 million (66% of U.S population) who are using health apps in USA. 
C) Find the penetration rate: “The Penetration Rate in marketing, is a parameter to show the rate of circulation of a product in a specific population.”
Find total number of potential customer(x)= 213 million
Divide total number of potential customer(x) by the total (y) i.e. 323 million total US population = x/y
= 213/323 (Health App users/total population)
D) Find potential market size (volume and value):
Market volume= penetration rate* # of target customers
=66%* 213 million
Market value= Market volume * Average Value
(Assuming average sale of $10 per user per month)