Hello
The sizes in patterns are the size of the finished item, if you work exactly to the given gauge.
A 40" body circumference could be called 49" around the chest or bust, it’s the measurement of the sweater all the way around.
It is not the measurement of the person’s body.
If the person has a 40" chest they likely do not wear a sweater measuring 40" but perhaps 42 or 44 or 46" depending g how much ease (gap between person’s and sweater) they like to have.
One way to decide what size to make is to get one of your own sweaters which you like the size of and measure it flat on the table, across from side to side, then double that measurement to give circumference needed. Look for a size that is close to this.
Switching from inches to cm can be done online, just type it Google.
40" for instance is 101cm and depending on amount of ease wanted might be good for a UK size 10 or 12 woman, a UK size 10 is about 34" bust but you need to add some ease, so 40" is probably suitable, taking into account that one styles are intended to be baggy or over sized and some intended to be figure hugging.
I’m size 10 ish and make sweaters 50cm wide, that is 100cm circumference for myself and they are generally a baggy slouchy fit. I make almost the same size for my son age 14 who would be a man’s xxs or xs in waist/chest, he’s pretty much an 8 to 10 woman’s size. Sorry I don’t know if this pattern is intended for male or female form/size so just trying to give you a rough idea of both.
Does this help, or have I misunderstood your question?