All of the advice above is sound. Length is not the only, or even prime, consideration. Neither is the presence of a keel.
Your intended use -- day-tripping, weekend camping, or longer expedition camping (a week or more) and the kind of waters you will be paddling -- narrow, winding streams, wider but quiet rivers and lakes, or white water -- should be your starting points.
Canoes are very adaptable. You can solo paddle in an 18’ canoe; you can tandem paddle down a long lake in a 15’ canoe. However, some canoes do some things better than others, and you should consider how and where you will be paddling when you choose your canoe.
Unless you are planning a lot of serious camping or otherwise carrying heavy loads, a 15' canoe will provide completely adequate length, even for tandem paddling, and will generally be lighter in weight than a longer hull. There is nothing wrong with a longer hull, of course -- other considerations come into play.
The shape of the hull is very important, at least as important as the length. Our 15' Old Town 50 Pound model, a bit wide and a bit flat-bottomed, is easily paddled solo or tandem, and is very maneuverable in narrow winding streams. It is easy to keep on course while paddling solo and its keel does not seem to impair its ready maneuverability when paddling through winding, narrow bog streams.
Our 16’ Old Town Ideal, with a hull that is a bit narrower and with a bottom a bit more rounded, is a faster boat, is a less maneuverable, and is a bit more tender. It also has a keel, but I believe it is the hull shape first, and length second, that more determine how it handles -- faster, more directionally steady, less agile. But a friend, an experienced free-style paddler who borrowed it at for a bit at the Maine Canoe Symposium a couple of years ago, spun it and turned it like a 10’ squirt boat. (I can’t paddle like that -- but the boat could.) And it could readily carry a bit more of a load than the 50 Pound boat, but perhaps not as much as you might think, because it is narrower.
Either canoe serves us well enough on the waters we usually paddle and for the purpose we usually paddle -- easy day trips with a picnic lunch as our cargo -- and we have chosen the canoe of the day depending mostly on our mood. I generally prefer the 15’ for solo paddling
Fairly recently we have acquired one of Jerry Stelmok’s 1889 models, a 15’ 4” canoe with a beam similar to the 50 Pound canoe, a less flat hull bottom, and no keel. It has a different feel than either of the other two canoes, but generally has the same functionality. When I get used to its better agility, I will probably prefer it for solo paddling.
And we have a 16’ Old Town Camper made of Royalex -- we use it when loaning to inexperienced friends, or when we expect conditions that may be abusive -- rocks in a shallow river or on the shores. Neither fast nor slow, and not particularly agile, it is a quite steady utility boat.
All other things being equal (though they rarely are), a longer canoe will be a bit faster, a bit more steady, will carry a greater load, and of course, will be heavier. If you are planning on a lot of poling, longer tends to be better. A narrower canoe will be a bit faster, a bit more tender, and a bit less agile.
The weight of a canoe should not be disregarded. If your paddling will involve car topping (virtually all of ours does), or portaging (only some of ours does), every time you lift the canoe over your head you will have occasion to consider whether a 17’ canoe was the right choice.
I have paddled longer boats, up to 20 feet, from time to time over the years. I was happy to use an 18’ canoe for a week-long camping trip in Montana on the Missouri river a couple of years ago, and was glad that such a boat was available for hire -- and also glad that I do not own such a canoe. And I am constantly tempted to get a shorter boat for solo paddling -- but I don’t really have the space to store another canoe.
For you, a longer canoe may be the right choice. You don’t explain why buying a shorter canoe would be a struggle for you.
The advice to try out some differing boats is very sound. When trying them out, keep constantly in mind how and where you will be using your canoe. Pay attention to all of the variables -- not just length.
