There is no 'magic bullet' answer to either question but there are simple guidelines but no hard and fast rules. Use common sense, experiment and frame the question to thinking like a designer.
Think in terms of lengths for the garment while considering the width of the fabric.
For instance if it is a jacket or blouse, consider the front and back and the sleeve (and most sleeves take about 3/4 yard. The front and back might be anywhere from 1/2 to 1 yard each -- don't forget facings, collar, cuffs etc. Ask yourself -- is the fabric wide enough so I can get both front and back in the same length? An example of this is our Cool Combos, where we put together a total of 3 yards of co-ordinating/harmonizing knits in a package. Some combos are 3 one yard cuts, others are 2 one and one half yard cuts. This gives lots of design options. One yard is usually enough for a front and/or back length and the other lengths can be used for a sleeve, cuff, neck etc. I recommend saving the scraps to use in other projects, especially stripes and dots which act as blenders.
Think like a designer. Professional designers buy sample cuts to test designs, most often in 3-5 yard lengths. When I am working on a prototype for a new Vogue pattern I usually take a 5 yard cut, planning ahead for design options and for mistakes. I don't want to run short if I need to cut an extra piece and frequently I end up cutting 2 different sleeves or collar styles to test out a design. In this option, you would come up with your own standard sample cut. I recently ran into a renowned SF women's clothing designer at a fabric show and she told me that to cut costs, she was no longer buying sample cuts in 10 yard lengths but rather in 3-5 yard cuts. (Sad for me as I love to scoop up her leftovers!).
Think like Diane Ericson. My good friend and design guru buys smaller amounts as she likes the challenge of 'not quite enough', and her style is to piece, insert, collage and use more than one fabric in a garment. Diane re-thinks and re-makes everything. She thinks nothing of combining bits of a thrift store t-shirt with an expensive fabric in the same garment. In this video she shows a group of fabrics she chose from our ArtBarn, how she will use them, and which fabrics she eliminated.
Think like Gwen Spencer, who works along side me selecting fabrics and making the prototypes for my Vogue patterns. It is always fun to shop the ArtBarn with Gwen to see what she chooses. Shown in the photo below are Gwen's picks from her last visit. She brings out a batch of fabrics and at the final decision bought 2 yards of each. Come visit our booth at the Puyallup Sewing Expo to see what she will make from these fabrics! We are in 600-606 in the Showcase.
While all fabrics in theory, can be washed or laundered, it is a practical idea to develop a system for handling fabrics as you buy them. In describing the fabrics in the marcytilton.com online store, I give loose recommendations for caring for each fabric posted. Here are some options to consider.
Katherine Tilton's Way: When Katherine chooses a fabric (especially knits), she tosses them in the washer/dryer using Express Wash and Normal dry. Almost always. Keeps things simple. When she was producing limited production clothing she worked out the percentage of shrinkage on a particular linen fabric. Then she sewed up a group of dresses WITHOUT pre-shrinking and after the sewing was done, tossed them in the washer/dryer to shrink to fit and give the fabric a soft washed effect. Mind you she didn't do this again as it was too complicated. Katherine occasionally washes and dries wool coatings or knits to 'felt' them for special projects but she doesn't do this as a general rule. Once a garment is made it is washed by hand or on the 'Delicate' cycle and air dried. The dryer is not kind to fabric wear.
Detergent or not? I do add detergentwhen pre-treating, it aids in removing residual fabric finishes.
Some fabrics like silk organza or wool coating don't respond well to laundering unless you want a more limp, wrinkled or shrunk up version of the original.
Most new fabrics will shrink the greatest amount in the first washing/drying. Dark colors can fade and look old before their time if put in the dryer. For this reason, I might put a black knit in the washer/dryer before sewing, but hand launder or machine wash gentle and air dry after sewing.
Pure white fabrics may have more residual shrinkage after the first laundering because they have never been dyed, so you might want to wash/dry white fabrics 2 or even 3 times before sewing.
Not all washing machines will have the same results. A customer returned a fabric because it had gotten funky when she washed it. I took a cut of the same fabric and put it in my washer/dryer with no change at all. Pre-treating fabric in a front loader vs a top loader can cause quite different results. Another good reason to test a swatch first.
Often I get questions about whether a wool or wool blend fabric will felt or not. Again, the only way to be sure is to make a test. Back in the day, I could be sure that tossing a cut of 100% wool jersey would result in substantial shrinkage, but many of today's wools are blended with other fibers or have been treated against shrinkage. Test. Test. Test.... Then Play, Play, Play.