Decide on the length and width you want for the back and find the difference between them by subtracting one from the other.
Cast on some stitches and knit a strip. You can guess how many you need and knit either a long narrow strip or a short wide one. Keep measuring the strip until the difference between its measurements is the same as the difference you have already calculated.
Work outwards in all directions until the back is the size you want.
For a sweater the front can be made in a similar way. For a jacket or cardigan start with a strip half the width of the back strip and build up the front from that.
Striped sleeves are easy to add, or incorporate a rectangle into each sleeve