Great ideas. For market sizing questions start with understanding what are the drivers behind the demand of the quantity you are estimating are. Then test these with the interviewer and structure your approach based on the drivers. In this example, there are also many answers but let's assume the following is the case: 

- Number of towers' main driver is cell phone usage plus there is a buffer of unused capacity in case of fluctuations in usage. 

- Inquire whether different networks need different towers or different providers need different towers. Let's assume the interviewer says you can simplify you do not need to consider different types of networks (eg 3G) or providers. If that is fine then the number of towers is simplified:

- - # towers= (# cell phones* average cell phone usage+ a buffer factor for unsed capacity in towers)/average total usage servable by a single tower 

- Then they might ask you additionally to estimate the number of cell phones in that country yourself, etc.

This is just one suggested simplified approach. My proposal is generally with market sizing questions test with the interviewer whether you are allowed to simplify by making him aware of the detailed option and suggesting ways to simplify. This way you are on the safe side.  If you are not using a calculator, even propose rounded numbers wherever possible. I hope this helps. If you have further questions, feel free to reach out personally.