What makes a market size good is as follows
1. It doesn't feel wrong - for example, if you estimate the number of consumer laptops sold in a year in the US and you get a number like 100,000 or 10Bn those both instinctively should set off alarm bells
2. You can book-end it in some way - for example you can say I know that there are 320M ppl in the US and say 150M households. It is fair to say 80% of households have at least 1 laptop so I can anchor to this number as some type of limit for my final answer
3. You followed logical assumptions to get there - example you did not assume that people on average have 5 laptops, or that 1 laptop lasts 10 years
All the best,
Thanks that's super helpful indeed!