Underneath is my approach. I am open for feedback.

Clarifying question:

What is meant by ‘market size’? Is it (a) the total money in the banking system relevant to individual banking OR (b) total revenue in the banking system because of individual banking solutions.

Presuming the answer is (b), I would use a top-down approach structured underneath:

There are five major services in individual banking market:

1. Savings and Checking Accounts : Total Revenue = Avg. Revenue per $1000 in the account x Avg. Account size (in $Ks)  X number of accounts

Number of Accounts could be estimated by segmenting the number of households in the country to:

0 Account: 10% HHs (Extremely Low/No Income)

2 Accounts: 40% HHs (Lower Income) 

4 Accounts:  40% HHs (Middle Income)

6 Accounts: 10% HHs (High Income)

2. Debit and Credit Cards Total Revenue = [Avg. fees from account holders + Avg. Transaction fee from Merchants x Avg. Annual Transaction/card] x Number of cards

Number of cards can be estimated with HH market sizing approach laid out above.

3. Loans Can be estimated with similar approach 

4. Investment and Wealth Management Can be estimated with similar approach

5. Insurance Can be estimated with similar approach

After laying out the structure, I would offer to move forward with Savings and Checking Accounts  if okay  with the interviewer.