The size question stops more people from buying than the scent question does. Here's a straightforward way to decide, based on what you're actually trying to find out.
2ml: for finding out if you like it at all
A 2ml decant gives you somewhere around 20 to 40 sprays, depending on the bottle's nozzle and how heavy your hand is. That's enough for several full wears, enough to smell the opening, the dry down, and how it holds up over a few hours on different days. It's the cheapest way to answer the only question that matters at this stage: do you actually like this, on your own skin, once the sample strip novelty wears off. If you're curious about a fragrance you've only smelled on a strip or on someone else, start here.
5ml: for living with it
A 5ml decant gets you roughly 50 to 100 sprays, enough to wear a fragrance regularly for two or three weeks. That's long enough to test it against conditions a 2ml sample can't tell you: how it wears in heat versus cold, how it holds up through a full work day, if you're still reaching for it after the first excitement wears off. Plenty of fragrances are great for the first three wears and forgettable by the tenth. 5ml is the size that tells you which one you've got.
10ml: for the ones you already know
By the time you're buying 10ml, you're not testing anymore, you're confirming. It's the size that makes sense once you already know a fragrance works for you, whether from a smaller decant or a bottle you've worn before. Roughly 100 to 200 sprays, it's usually the best cost per ml short of a 30ml, and it's a useful travel size: well under the 100ml carry on liquid limit, so it goes in a bag without a second thought. Good as a signature scent backup, or for taking a favorite somewhere without checking a full bottle.
If you're choosing between several
You don't have to pick one fragrance and commit. Most people who build a decant collection end up with a handful of 2ml and 5ml sizes in rotation rather than one 100ml bottle they get bored of by month two. If you want to try several profiles at once instead of buying 2ml samples one at a time, our discovery sets bundle a few together for less than buying them separately, a fast way to figure out which direction to go before committing to anything bigger.
Every size we pour, from 2ml up, is hand-poured to order in Regina, and orders over $55 ship free to the provinces, so testing a few sizes at once doesn't come with a shipping penalty on top of the guesswork.