Hi there,

First, you should be clear on the scope of what you actually need to calculate. Only then you can set yourself on the approach. In general, I'd suggest you a bottom up approach however:

As I don't know the exact assignment you get: I imagine you get to calculate (1) the number of individuals who buy giving that they see the and (2) the ad is for a specific type of products (e.g. pet food) (3) in a certain area (e.g. the USA).

Now, I would define who will actually see the ad. In my example, consumers using Instagram living in the USA who are looking at dog related content. We could say all people who have dogs, and some who want to get one, or are heavily interested.

So we need to find: No. of Instagram Users in the USA x % of people looking at dog content

Now we need to deep dive on both factors with assumptions

No. of Instagram users: e.g. 80% of Americans between ages 16-40 (you can differentiate this further by age brackets if you want to)

Share of US Population between 16-40 with a dog: Let's assume 20% of people have a dog (or have one in the family they are in, they don't need to be the actual owners), and another 5% are interested or want one (you can verify this by guessing the number of dogs per person e.g. in your extended family, if you are not an outlier with many or no dogs)

Now you have the number of people seeing the ad. If you want a market size in value (i.e. $), further assume the share of people who buy given that they see the ad and their average shopping basket.

Now plug in the numbers and you are good to go :)

Hello!

I would always recommend you to give it a shot and then post it here, this would help you the most. 

Given that market sizing cases were the topic of many questions in this Forum, I developped a market sizing case, that you can find for free in PrepL´s library

https://www.preplounge.com/en/management-consulting-cases/candidate-led-usual-style/intermediate/market-sizing-new-startup-launch-baby-strollers-in-the-us-244

Furthermore, you can find a detailed market sizing (a complex one integrated into a case) in my latest case: 

Swiss manufacturer market assessment, profitability and scenarios: https://www.preplounge.com/en/management-consulting-cases/candidate-led-usual-style/advanced/swiss-coffins-the-death-business-245

Let me know if you have any doubts with it, it contains a detailed explanation and methodology about how to solve this specific example, but you can extrapolate to many other market sizings!