You are probably referring to the penalty arc which is at the top to the penalty area. It is not exactly a half of a circle.
Prior to the taking of a penalty kick all players except the kicker and the opposing goal keeper must be outside of the penalty area and at least 10 yards from the penalty mark. The penalty mark is only 8 yards from the edge of the penalty area! So an extra bit of distance is added to show the players where they must be.
it is half of a men soccer field
No. Soccer fields only have the lines where the field ends and the boxes close to the goal keepers and a circle when the 1st half and 2nd half starts .
half way line
Yes it has 42 and a half
illegal. player must be completely off the playing field.
This is really not possible to answer since there is no single size of a soccer field. FIFA publishes a range of acceptable values.
it matters in what country in Brazil they do but in America they don't