If a sweater is too small it is possible to block it larger, not too much, but depending on the materials and the pattern it is possible to make the item a size or two larger. One other way is to add a small panel or gusset between the front and back at each side. This works especially well if you are knitting a sleeveless garment such as a vest. The armhole will be enlarged so if you are making a sweater with sleeves the sleeves will need to be adjusted.
If, alternately, the garment is too large you can make it smaller by treating the pieces as fabric and cutting it. This is not as radical as it sounds, Fair Isle knitters and Norwegian knitters do this all the time to make cardigans and add sleeves. Fair Isle knitting and Norwegian knitting is completed in the round without purling and the front of the garment is cut and button bands or edges are added. You can use a sewing machine to run a row of stitches through your knitting to hold the stitches when you cut. However depending on the material used this stitching may not be necessary. Materials that naturally felt such as wool may not need the sewing machine stitches, but cotton or silk will definitely need to be stitched with a machine to keep the stitches from “running”.
Back to adding additional material to make the garment wider. If it is a sweater or vest, place the garment on waste yarn and take it off the needles and then try it on. Take a measurement of how much larger you need to make the garment and divide it in half. Here is where it is possible to creative you can make a rectangle in a pattern that is the same as what is in the garment or you can make a rectangle that is a bit different. For example,
This little bit of extra knitting preventing me from having to rip out the vest and start again and kept this vest from becoming another unfinished object in my basket.