The first time I went to this new barber I told him I wanted my hair short on the sides and half the size of my hair length on top.It came out perfecrt then when I went back to get the same hair cut it wasn't the same he keeps making it too long on top should I tell him to make the top (the size I want) centimeters or milimeters long or what.any advice

If he leaves it too long the next time, just tell him to trim a little bit more off. I would rather have it left too long than too short, as you can't do anything with it when it's too short but wait for it to grow again!