Had the same issue. In some reason by default game set too big screen resolution. To solve the problem you have manually fix screen resolution in config file.
Open file ~/Documents/Paradox Interactive/Crusader Kings II/settings.txt
Modify graphics -> size and put some lower resolution value
graphics=
{
size=}
{
x=1650}
y=1050
....
The above resolution works for 15'' and for 13'' MacBook Pro use x = 1440, y = 900