I am interested in knitting a sweater skirt and perhaps a sweater dress; my question is whether or not they have a tendency to stretch all out of shape after you wear them? Also, as a side question, do knit bags also stretch out of shape? Thank you for your help.
As long as you use a good quality yarn, the only thing that really matters is the fit. If you get the fit right, I can attest to the fact that a skirt will not change shape, even after sitting a long while. I recommend a near body fit for the yoke area or the 4-5 inches from the waistband, the you need a minimum of the widest measurement of your hip area, plus maybe half an inch. If you check out the lattice skirt I made from one of Gwen Bortner's beautiful patterns in the Skirt Issue of Knitter's Magazine on my Ravelry page (Raptorhaven), this is one of the skirts I am referencing. I sit all day at work and it never shows a stretching of the hind area because the fit is perfect.
Whether or not the skirt will stretch, depends on1) type of yarn2) pattern stitch/gauge
Acrylic yarns stretch a lot. Sturdy cottons, linen and wool do not.
Delicate/pure wools may felt in areas that get a lot of friction (the seat).
Loosely knit/lacey patterns will stretch more. Doing such patterns in pieces, then seaming will help control this some. And/or keeping the top/waist section of the skirt tighter and having the lace/pattern only on the lower portion/ruffle/hem.
Garter stitch and cables won't stretch as much.
Knitting on the bias or sideways won't stretch as much.