The honest answer is that there's no wrong size — only a mismatch between what you bought and what you actually needed it for. Here's how the three measures tend to get used in practice. (A decant is a smaller measure of the original fragrance, prepared in an atomiser.)
2ml — the test
Two millilitres is a smaller size suited to trying a fragrance over several wears. It's not really meant to last — it's meant to answer one question: do you actually like this, once you've worn it a few times and it's had a chance to develop on your skin? That's plenty for a proper test. Over a few wears you'll get past the opening, see how the heart and base sit on you, and by the end you'll know whether it's worth going further. If you want a method for that, see how to try a fragrance properly.
This is the size for curiosity — a fragrance you've read about, one a friend wears, something that caught your eye in a shop but you're not ready to commit to a full bottle before knowing if it actually suits you. It works well for exploring the designer or niche ranges.
5ml — enough to actually live with it
Five millilitres gives you more time to wear a fragrance in different settings before deciding whether you want the full bottle — enough that it stops being a test and starts being something you're genuinely wearing. It fits in a jacket pocket or a wash bag without any trouble, which makes it a handy size to take on a trip, or keep at a desk for the days you forgot to put anything on that morning.
If 2ml told you that you like a fragrance, 5ml is where you find out whether you actually reach for it — whether it becomes something you put on without thinking, rather than something you remember you own.
10ml — the one you already know
Ten millilitres is the largest measure, and it's really for a fragrance you've already decided on. Maybe you tried it at 2ml, lived with it at 5ml, and now you know it's staying in the rotation for a while. Or maybe it's a fragrance you already know well and you just want a lighter, easier way to carry it without a full-size bottle taking up the space.
A simple way to think about it
If you're asking "would I like this?" — start at 2ml. If you already know you like it and want to actually wear it for a while — 5ml. If you know exactly what it is and just want more of it, in a smaller footprint — 10ml.
You can always move up. If you start at 2ml and like what you find, 5ml or 10ml of the same fragrance is there when you want more. The measures aren't a ladder you have to climb in order — they're just different amounts of the same honest answer: try it, on your own skin, for as long as it takes to actually know.