I'm not sure you're asking how to determine the size of the wireless network in the US or how to approach this particular case. So just in case I'm going to address both:
Always good to ask some questions to have a feel for it:
1) Is the service we provide in Asia the same service we will provide in the US or do we need any upgrade? (this could help in determine if further investments are required)
2) Why did you choose the US market? (this could give us some intel on growth rate or size)
3) What is our client looking for with this deal? (real objective: breakeven; market share; volume, etc)
Then I would look at 3 main areas: External Conditions; Internal Capabilities; Financial Opportunities.
External Conditions: Market (size, growth, trends, segments, barriers to enter; etc) & Competition (market share, prices, etc)
Internal: Client (cost structure; network; swot) & product (how does our product differ from comp.)
Financial Opportunity: Breakeven analysis (if this applies after the inicial questions)
Market Size analysis: If the interviewer asks you to determine probably the easiest way to do it would be:
# households with wireless network = (US Pop/#of ppl in a family) * %households with internet * %households w/ wireless network
Here I'm assuming just for households but if we are talking about wireless network in smartphones I would had that to the above equation.
Challenges upon entering were covered in the "Market" branch on the "barriers to enter" - basically: Inicial investment; brand recognition, regulations, strong competition, etc.