I once determined I needed six boats to serve all circumstances well. My first objective was to get one boat that best served our most important pursuit well, paddle camping.
I now own a 21' tandem, nicknamed Queen Mary by friends for its "maneuverability", but I had rented and borrowed many others before settling on this boat. I figured there are always at least two of us and I wanted a boat for paddle camping so the larger boat does very well what others can't (e.g. carry large loads, good sea kayaking or rough water, tracks well, streamlined so easy to move along with comparatively little effort ... long paddles less tiring as a result) but is overkill for lots of my paddling.
True, a short boat is more maneuverable but even the 21' has served us well for a variety of rivers, lakes, creeks, bays, marshes. Sure, sometimes we backpaddle a bit to get back in more navigable water but we've deemed it a small price to pay for all the other features.
The real short boats are most suitable for white water. Our boat can get us in trouble above class I so we'd either avoid anything rougher or portage if feasible.
My next boat will probably be a single and I think about 12', as lightweight as possible, with a narrow beam and under hatch storage. I like the longer boat for its tracking and a streamlined boat paddles easier and less tiring over long distances. I like long distances, they increase the opportunities for explorations. The less tiring aspect also means less wear and tear on your body over a long time. Plan now for retirement ... you'd not like being riddled with tendenitis and arthritis just when you have the time to get out whenever you want. (Lightweight paddles also pay off in this regard.)
A shorter boat can mean less weight and that serves the above purpose but keep in mind, the "efficiency" is determined by the ratio between length and beam, along with the boat's lines, and there is only so narrow that a boat can get so short boats with the same beam as a longer boat will likely be more work to paddle, unless it is markedly lower in weight.
My suggestion is to try out a few boats in your range of sizes to help with your decision. Keep in mind that a bigger boat can do everything a smaller boat can, 'cept for that whitewater thing, but it can be dangerous to take a small boat to do a bigger boat's job.
One other thing, comfort is important, especially over longer distances. A paddler with a wide beam may not be very comfortable in a narrow boat. Seats matter considerably, too. Get a boat with good seats or adapt the seat to give good lower back support and you'll enjoy almost any paddle more.
Can't recommend much in NH except Umbagog and Great Bay. Massachusetts, Maine and Florida are my favorite paddle places ... Mass. because there is so much that is convenient.
