Is there a formula for making a pattern fit a larger size? I bought a pattern which to a casual perusal appeared to be my size but when I had the time to read it realised the larger sizes were for a man's sweater rather than a ladies cardigan. Hate to think I've wasted nearly three pounds.
I think you would have to draw it out on graph paper with a square for every stitch. Then you can work out how many more stitches you need. If it's only a bit small you might get away with knitting it on a size larger needles
Are you saying you have a man's pattern rather than a Woman's cardigan?
If the pattern is plain stocking stitch you could probably do it with regard to the number of stitches per 10cm as on a tension square for the yarn you are using.
If it's a fancy pattern is concerned it's going to be a bit more complicated as you will need to have enough stitches to complete any pattern that involves a sequence of stitches.
Look at other woman's patterns your size to get an idea how many stitches you need for that garment. The important bit to get right is the width, ie number of stitches. The length of the body or sleeves as they are rows is much easier to adjust.