I'm currently working as a pre-MBA associate at a leading Silicon Valley VC firm (nearly $1B in current AUM). Happy to take any questions about working as a VC associate, recruiting, etc.
In terms of my background, I've worked in both business operations at a startup and tech banking at a bulge-bracket firm. Studied CS undergrad from a target school.
Market sizing is as simple as price per unit * units sold. As a simple example, let's say you're asked to size the market for wearable baby monitors. The average price for a baby monitor is $200 (based on existing products). There are 4M babies born in the US each year per the US Census Bureau, and assuming that 25% of households can afford to and are willing to get a baby monitor, that leads to a $200M addressable market. Sometimes you'll have to go into more detail around different segments and assumptions and do a rollup to fully size the market. Cab hailing for example is a more nuanced example. You would have to research or make an assumption about the average spend per ride * number of rides per cab * number of cabs. I'd calculate this city by city for each major city and roll them up. As a sanity check, you can usually compare your results to existing offline market sizes, which are published in articles and research reports. A quick search for the taxi and limo market shows it's ~$10B in annual revenue.
As for modeling, you're correct in that all the high finance skills you learned in banking aren't directly applicable to early stage VC. There's some in growth equity but still not nearly as much as you would do in PE or banking. That being said, VCs are biased towards analytical people with pedigrees. Most associates I know in VC, even in early stage funds, are predominantly ex-bankers or consultants.