generally speaking a 'part' is anything you want it to be, any size.
say each part is 1ml. or 5ml, 10ml, 25ml or even 1 gallon.
as long as the 'parts' are equal to each other.
if your 'part' is 10ml, and your ratio is 2 to 1,
2 parts(10ml each) plus 1 part(10ml) you will get 30ml mixed...