Glad to see another large-scale fan :) - I feel rather out of place on
the modelwarships.com forums with all the 1/700 modellers, there don't
even seem to be many 1/350-1/400 fans out there.
AFAIK there are three really large (non-sail) surface warship models
available as mainstream injection-moulded kits. The 1/200
Yamato/Musashi has already been mentioned, the other two are the
Matchbox/Revell 1/72 Flower-class corvette and the Lindberg 1/125
Fletcher-class destroyer (aka "Blue Devil"). Both of these are around
3ft long.
(I've also heard that Doyusha made a Yamato/Musashi and Shokaku
(carrier w/Yamato-class hull) in 1/250, but I've never seen any of
these.)
The latter is infamous among ship modellers, as you said "no
scratchbuilding" this kit isn't really an option as it needs a LOT of
scratchbuilding work and superdetailing to produce a decent model (ask
Jeff Herne...)
The former (Flower-class) is much better than the Blue Devil in terms
of accuracy and moulding quality, but the smaller parts such as
armaments and fittings are -very- lacking in detail. I'm currently
building this kit and have put a lot of work into improving it with
scratchbuilt styrene and metal parts.
IMHO this kit really needs superdetailing work (either scratchbuilt, or
aftermarket parts from "Great Little Ships") but built OOB it is still
a nice model of an unusual subject.
If you want a big, large-scale naval model which doesn't require
scratchbuilding or aftermarket to produce a detailed, accurate result,
your best option is probably the Revell 1/72 Type VII U-boat (or the
Gato-class when it's released later this year). Both of these are also
around 3ft long, submarines may not be your area of interest however!
There are also quite a few kits of torpedo boats in 1/72 scale (most of
which are still widely available in shops and online) and a
surprisingly large range of WW2 and modern warship kits in everything
from 1/125 to 1/225 from Lindberg, Nichimo, Pyro etc. Many of the
latter are out of production and hard to find even on eBay however.
(Look through Rajen's Ship Kit List - http://www.quuxuum.org/rajens_list/rajen.html)
Ones still reasonably easy to find are the Lindberg LCT and LST landing
craft, Glencoe 1/225 USS Oregon (predreadnought) and Nichimo 1/200
Japanese WW2 destroyers.
These aren't particularly big as regards physical size (though larger
than virtually everything in 1/700) but the large scale makes them
easier to work with (especially regarding scratchbuilding and PE).
Large-scale models can be at least as intricate and tricky to build as
1/700 if you start adding scratchbuilt or aftermarket detail (the main
components and fittings are larger, but they have lots of tiny parts
which are at least as small as 1/700) but IMHO the greater level of
detail makes the finished result more interesting.
EDIT: Also forgot to mention the Trumpeter 1/200 kits. There's quite a wide range of these,
the kits aren't bad, are good value, and widely available in both the
US and Europe, though the subjects (modern Chinese warships) probably
won't appeal to a great number of modellers outside China. One of the
kits is actually a WW2 Soviet
destroyer and can be converted to its original configuration without too much trouble.