Why don't you take a look at the myriad resources already out there on market sizing? Perhaps the hundreds of Q&As with examples?
Or, one of the practice market sizings that have been created: https://www.preplounge.com/en/management-consulting-cases/brain-teaser/intermediate/taxis-in-manhattan-market-sizing-229
You'll learn a lot more from this by trying it out yourself first! Why don't you post your approach here and we can provide commentary?
How to approach market sizing
It's very simple: Do the approach the is the easiest for you given the question.
Are they asking you to estimate something where you don't even know where to begin from the top (maybe you have 0 clue as to the market size of the industry, the GDP of that country, etc. etc.)? Then do bottom-up!
Alternatively, does it seem impossible to do a realistic from-the-ground-up estimation of something (perhaps it requires just far too many steps and assumptions)? Then do top-down!
Fundamentally, you need to take the approach that just makes the most sense in that circumstance. Quickly think about the key assumptions / numbers required and whether you 1) Know them or 2) Can reasonably estimate them. If you can, go ahead!
He's a Q&A for a great market sizing question here asking to estimate # of electric charging stations in a city in 10 years:
This one could be answered top-down (as I did) by estimating population of the city, # of drivers/ cars, etc. etc.
OR, it could be answered bottom-up by estimating # of stations you see per block (or # of gas/petrol tanks), % increase this might be over time (or # of EV stations that would be needed per gas tank given EV stations take 10 times as long), and # of blocks you'd estimate the city to have.