Code option: 
TabPane pane = new TabPane();
pane.setTabMinWidth(200);
pane.setTabMaxWidth(200);
Css Options 
.tab-pane{
    -fx-tab-min-width:200;
    -fx-tab-max-width:200;
}

Old solution:
You have to set -fx-max-width for .tab > .tab-container > .tab-label  There are 2 options: 
1) Set the default sizes for label and not tab (selected tab will be a little bit larger with x button
  in it):
.tab-pane > .tab-header-area > .headers-region > .tab > .tab-container > .tab-label {
    -fx-min-width:200;
    -fx-pref-width:200;
    -fx-max-width:200;
}
2) Use your css and some smaller value for labels(This way all tabs
  will be always 200 px):
.tab-header-area .tab{
    -fx-min-width:200;
    -fx-pref-width:200;
    -fx-max-width:200;
}
.tab-pane > .tab-header-area > .headers-region > .tab > .tab-container > .tab-label {
   -fx-min-width:175;
   -fx-pref-width:175;
   -fx-max-width:175;
}