The most important things you need to look at are:
- What is their raising range from all positions
- Do they play draws slow or fast
- Do they have a fold button
- Are they sticky
- How do they respond to 3bets both in and out of position
- Do they love or hate money (if they love it, you can bluff them when flushes complete, or over cards, etc).
- Do they raise to protect or do they raise as a semi bluff
Then use these things to your advantage. The most important thing you should be noticing is what is their raising range. If they raise any two broadways from all positions, you know you can easily 3bet them light from the blinds. If they're foldy to 3bets, you can widen your light 3bet range against them. If they're sticky pf, obv narrow that range.
I don't think bet sizing is as important as their ranges, but some players do have some noticeable bet sizing issues. For example if they 3x raise from early position, it's usually a multiway hand and they're trying to build a pot. Those player are easily exploitable by 3betting them light (especially if they get multiple callers and I'm in the BB with KTo. Those squeeze plays sometimes are like finding dollar bills on the sidewalk and sweeping them up).