1) first of all every market size can be a bit different, but the most important thing is that you should be able to understand and figure out a specific driver...
for example if the product/ service is connected to the average use from people... you can use the population as main driver; instead if it is something connected to a geographical distribution (eg number of taxi or newsstands in New York City) you can think about the number of blocks and then position a reasonable number of items suing the blocks
2) once you have figured out the main driver, you can think about a segmentation (eg by age brackets...)
3) then you need to make reasonable hypothesis to convert a number (eg number of people or number of shops...) into a yearly revenue figure. Important here to remember that the market size is usually an yearly figure and not a monthly one...
lets try to make as much granular thinking as possible, while not exceeding 15/20 minutes.
its usually a good strategy rounding numbers
let me know if I can help you more!