Let's try to estimate this for the city of London

I start with formulating the approach in the form of an equation

Step 1: Raincoat market size ($) = City population(#) * (% of pop. that feels the need for a raincoat) * (Avg number of raincoats owned) *(1/Life of a raincoat in years) * (Avg. price of raincoat in $)

Step 2: Input my assumptions

London city population - ~10M
% of pop. that feels the need for a raincoat - Since London has high rainfall, I will assume a large % of the population would buy raincoats (~80%)
Avg number of raincoats owned - I would assume ~2 considering high rainfall in London
Life of raincoat - ~4 years
Avg. price of a raincoat - ~$50

Step 3 - Calculate using my assumptions
Market size ($) = $200M

Step 4 - Do a sanity check
Dividing $200M by the population of London (10M) gives $20 which means each person in London spends on average $20/year on raincoat purchase. This does not seem far fetched.

Conclude - Considering my assumptions, the market size for raincoats in the city of London should be in the vicinity of ~$200M