I printed out the free pattern. Then got all the measurements I need written down on paper. Look at the printed pattern for picture of sweater form. I wrote in all the measurements I will need on my picture and also converted all of the measurements given to inches. This is a good place to start and free so far. Now decide on yarn choices and color options. I am knitting it as given. If I want to make changes I will do it the second time around.
If you need a larger or looser fitting sweater now is the time to figure out the changes. The largest measurement I see given is just under 50 inches around. Many of us will want a looser sweater. Figure out how big around you want yours.
If knitting at gauge every 21 stitches is another 4 inches or so. The graphs are 12 stitches wide. So an additional cast on of 24 stitches gives you a fat 4 inches extra and 2 extra repeats of the sweater graphs. If that is correct then an additional cast on of 48 stitches should give you about 8 to 81/2 inches additional sweater. Also giving you 4 additional graphs. One each for the left side back and front, and for the right side back and front. Still easily divided up and markered off.
Remember if you add width it will move your sleeve down your arm by that much too. So if you add 8 inches to the sweater it will move the sleeve down 2 inches. Your sleeve would need to be 2 inches shorter. Now someone that is following this along see if my figures sound right and add a comment to this post. I don't think the neck would need much adjusting, depends on what you need personally.