A paddle we find comfortable can enrich our paddling experience enormously… but because we can adjust to just about any close-enough length of canoe paddle, and because we tend to grow to love what we know… preferences vary enormously… so nothing really beats trying lots of paddles, and trying different lengths with a genuinely open mind - but we can generally get into the right ballpark even on dry land.
HOW LONG IS LONG ENOUGH?
We don't need to get obsessive about paddle length, but it's surprisingly easy to give someone two paddles of identical length… and find one is slightly too short… whilst the other is, if anything, slightly too long!
In deep water, pretty much the only thing that really matters about paddle length is the shaft length. That's the length of the paddle that's out of the water whilst we're pushing the boat past the blade.
If we're routinely in shallow water, we might compromise a little on shaft length as well as blade style, but as a rule… our ball-park for "how long is long enough" starts with what's good when there's enough water to paddle.

Paddle-blades only really "lock" on the water if they're fully submerged before we load them. Anything less tends to let air get sucked down behind the blade. We can often hear this happening, and the results can be seen in swirling water even after our paddle stroke has finished… but that's always likely to happen with a paddle shaft that's too short!

SIZING A CANOE PADDLE SHAFT…
If a paddle is going to feel right for us, it needs to work with our idea of comfortable hand spacing. This needs be whatever is going to become “normal” for us - which is not easy to say if we're just starting out!
Our style will end up evolving to suit our paddle - so if in doubt, just get comfortable with a grip which is wider than shoulder-width but no wider than we’d want to do an old fashioned chin-up or press-up.
That's great. Now we just need enough "extra" shaft length below our bottom hand to get the blade completely in the water (and no more) at the start of each forward stroke.
Do we all have the same reach as we wind up for each stroke? Of course not - and if we switch from sitting to kneeling, our reach is likely to extend further - which is why bent shaft paddles (typically used from seated) tend to be slightly shorter than straight shaft paddles (often used from a kneeling stance).
If we tend to swap between kneeling and sitting, we might go on the short side if we’re mostly going to be cruising along but perhaps on the long side if we’re mostly going to be kneeling (especially on white water).
Our preferred "extra" paddle-shaft might change as we paddle more, but if we’re sat on a seat, or even kneeling on a thwart, our lower hand is likely to be at least 8” / 20cm clear of the water (and maybe 12” / 30cm if we’re a kid).
SO WHAT'S THE MAGIC NUMBER?
To work out what paddle to order, we simply add up what we require. For example, we may want:
Shaft for a comfortable grip: 28” / 71cm
Extra to the top of the blade: 9” / 23cm
Modern blade: 20” / 51cm
Total of the above: 57” / 145cm

What if we walk into a store with a standard 20” / 51cm blade in mind but then fall for a traditional blade that’s 26” / 66cm long? Yup, we just add the extra 6” / 15cm and instead of getting a 57” / 145cm overall length paddle, we get a 63” / 160cm overall length paddle.
Paddle-shaft diameter also matters.
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SIZING FOR A YOUNG CANOEIST?
Does anything need to change if we’re sizing a paddle for a youngster?
Well, our younger-paddler's most comfortable grip on a short, lightweight paddle might be a mere 19” / 48cm… but if that youngster is struggling with a heavy blade, the narrowest sustainable grip might be 21” / 53cm.
Adjusting for smaller paddlers can be especially important in a canoe sized for a larger adult. Our young paddlers commonly struggle to get their bottom hands out over the side of the boat and close to the water.
If we can't get out bottom hand below the gunwale, the paddle shaft ought to be longer… but the blade weight is then at the end of an even longer lever.
As a rule, good paddle balance is hard to manage with smaller paddlers adapting to boats designed for larger paddlers. Paddles tend to be blade-heavy. Reducing blade size is great for multiple reasons, and reducing blade-weight can help significantly. Of course, beyond a certain point, this puts us into premium paddles.
We can only scale paddles proportionately from young children to larger adults if the boat being paddled (or the distance from the bottom hand to the surface of the water) is also scaled!
MORE THAN ONE PADDLE?
If we're going to own or carry a spare paddle, we could just go for another that's identically-sized to our primary paddle… but for a variety of reasons, we might choose to go shorter or longer.
One perfectly good reason is because we want a spare paddle that will also work for someone else: a tandem partner, a family member, etc.
Another is for an upwind (smaller) and downwind (larger) paddle.

We may also find we prefer a shorter paddle-shaft when we're seated and a slightly longer paddle when kneeling. This is quite commonplace, especially if we rotate less and/or use a bent-shaft paddle when we're seated.
In practice, we can often adjust pretty much seamlessly to a paddle shaft that is slightly different to our "normal" paddle… but nothing really beats trying lots of other people's paddles to find out!