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.