If you have a pattern in a different size than you want to knit, then you should consider to use another yarn which goes to a different needle size. To make a smaller version you then will have to choose a thinner yarn, and the opposite if you want a larger size of the finished project.
When you have chosen yarn, check out the recommended gauge for it. Then you can use it to find which of the sizes in the pattern that will give the wished circumference. If, for example, the recommended gauge is 22 stitches per 10 cm, and you want the finished garment to have a circumference of 100 cm, then you will have to do a minor calculation. First find how many stitches there is per cm; 22 sts / 10 cm = 2,2 sts. Then multiply it with the circumference; 100 cm * 2,2 sts = 220 sts. Compare your answer with the stated number of stitches the different sizes in the pattern. Now you are able to choose the size that comes closest to desired result.
To knit the design Hjemover in another size
Here is an example where the children’s sweater Hjemover is turned in to a jacket in female size M. To transform the size 8 YO to a fitted size M both yarn and needle were changed. This jacket is made with DK weight yarn and a 4 mm needle instead of fingering weight yarn and a 3 mm needle. Also the length of both the body and the sleeves are adjusted to reach the desired length.
A nice result, or what? And besides this pattern gives a really good opportunity to use up leftover yarn.