People tend to buy the biggest bottle available and then quietly stop carrying it. Capacity is a trade-off against weight and whether it fits the places you actually put it. Here's how the three sizes differ in practice.
24oz — the one that goes everywhere
The lightest of the three and the only size that reliably fits a standard bag pocket and most car cup holders. If your bottle needs to travel — commute, gym, errands — this is the size that gets used rather than left behind. The trade-off is refills: you'll fill it two or three times across a day.
32oz — the everyday middle
Big enough that a desk day needs roughly one refill, small enough to still carry comfortably. This is the size most people settle on once they've owned a couple. It's the sweet spot if you want one bottle rather than one per situation.
40oz — the desk anchor
A 40oz tumbler holds most of a day's water at once, which is the entire appeal. It is also heavy when full and won't fit every cup holder or bag. Think of it as a stationary bottle: desk, kitchen counter, passenger seat. Excellent at that job, awkward outside it.
Lids matter more than you'd think
A FreeSip-style spout lets you sip upright or tilt and drink, which suits walking around. A flip or slider lid is faster to open one-handed and easier to clean, which suits a desk. Straw lids are the easiest to drink from without looking, and the most fiddly to wash.
The honest answer
If you're buying your first, take the 32oz. If you already have one and it stays home, add a 24oz for carrying. Buy the 40oz when you know it's going to live in one place.