To determine market size, you will need to look at potential customer data or revenue/transactions each year.

You will need to:
1. Define your target customer and audience
2. Estimate audience size and target customers
3. Determine growth rate if possible taking into account organic and churn
4. Calculate the market size for both revenue and products/services sold

In terms of resources:
1. You can use bottom up data e.g. if opening a men's barbers then x number of men have x number of haircuts per year x average cost (in your target geography)
2. Top down data - get data from industry associations and apply your assumptions

This process isn't complicated but it is time consuming when done comprehensively.

You can also find freelancers on various sites who will do all this background research for you and who sometimes offer a secret shopper service calling competitors to get data such as for pricing.

Good luck!