Not So Big

What is too big or too small is a subjective question, that is dependent on individual attitudes and family needs.  To further complicate matters, just making a house small doesn't necessarily make it use less materials or energy, because shape is also important (more...).   Instead think in terms of "just the right size", which means that you'll  regularly use all the space you build (more...).  

The floor plan is the most complicated part, because in addition to building just the right amount of space, you need to make sure the house flows right, that you've allowed for passive solar etc. (more..)

Also for lots of ideas on how to make a house "not so big",  check out www.notsobighouse.com.

Reclaimed materials

There are three class of reclaimed materials, each one with its own level of difficulty.

Reclaimed & Re-manufactured

These are products that someone else has reclaimed and is selling in a form ready to use.  These are the easiest to use, and the most readily available. Most of the available wood is of very high quality.  Using wood with more "defects" is more challenging from both a functional and aesthetic perspective.  There is a design aesthetic, based on the Japanese idea of  "wabi-sabi" where "defects" become "character".  When holes and cracks are not functionally desirable, glues, epoxies and fillers can be used to make a very functional finished product, although there is a significant learning curve for the carpenter to do it efficiently.

Finger-jointed material is usually made from manufacturing scrap, and it another readily available product.  Both trim lumber and 2x4s can typically be found.

Salvage

These are products that someone else has removed, but they are typically in the "raw" state and need some cleaning up to be used.  Used flooring and trim are the two most common products, although old sinks, tubs, faucets, lighting fixtures, doors, windows, stained glass, cabinets and cabinet hardware are also used.  These materials require much more processing than other materials, although often only unskilled labor is needed.  Flooring must be de-nailed, and the edges must be scraped clean of finish.  Other materials require similar cleanup.  Sometimes salvage materials from non-residential building can be reused in homes, although they are often not the right size.  Examples are: cabinets, doors, stained glass, bleacher boards, wainscot, old bowling alleys.

Although most salvage materials are of the "antique" variety, its is also possible to salvage construction materials such as structural lumber, "one by" sheathing lumber, and bricks.  Broken (or "rubble") concrete in larger chunks can make nice walkways, or in smaller chunks makes short retaining walls.

Found materials (trolling)

Salvage is as much a creative state of mind as anything.  In the beginning you tend to stare at materials and either think they were inferior or not be able to think of what we could do with it. As you start doing it, your list of mental ideas gets bigger and bigger until we could see uses for almost anything (although not necessarily in your house).  This isn't likely to happen unless you've got a lot of experience, but it does indicate that there are many more possibilities than are obvious.

Construction creates a lot of scrap, and that scrap creates a lot of opportunities.  In the future, rather than throwing this stuff out, we will want to make useful things out of it.

Sustainably Harvested Materials

It is easy to use certified sustainably harvested wood (FSC), although availability often limits how much you can use.  There are currently a number of competing certification systems out there, including SFI, which is the timber industry's own self-policing, but none are as rigorous as FSC. There are many technical differences, but the main philosophical one is that FSC is an independent organization founded on environmental principles.  The debate as to whether this is relevant will undoubtedly rage on few quite a few more years.

Durable Materials

When given a chance, chose materials that will last a very long time.  This is often a very complex choice, since a durable material may be of dubious environmental merits.  In the case where lack of maintenance could cause the building to fail, its probably best to stick to the more durable even when their environmental merits aren't the best.

Other Issues

Recycled content is always an environmental choice, but a material that can itself be recycled or reused is even better.   Unfortunately many recycled content materials are composites that are difficult to recycle--still getting two uses is better than one.