// Setup a new relation editor with an upper hard limit of 10 items. Items past this amount will be automatically
// removed by GridFieldLimitItems (setup in this relation editor).
$gridConfig = GridFieldConfig_LimitedRelationEditor::create(10);
$gridField = new GridField('RelationName', 'Relation Title', $this->MyRelation()->sort('Sort'), $gridConfig);
$fields->addFieldToTab('Root.main', $gridField);

// Setup a brand new limiter...
$limiter = new GridFieldLimitItems(10); // Limit to max of 10.
$gridConfig->addComponent($limiter);

// ... or get from an existing configuration.
$limiter = $gridConfig->getComponentByType('GridFieldLimitItems');

// Setup the limiter with a few extra options.
$limiter->setNoteBelow(); // Ensure note displays below grid field instead of on top (default).
$limiter->setRemoveFromTop(true); // Removes excess items from the top of the list instead of the bottom (default).

// You can even setup a callbacks to prevent any manipulation from taking place under certain circumstances.
$limiter->onBeforeManipulate(function(GridField $grid, SS_List $list) use($disableLimiting) {
	// Will prevent manipulation if you return false from this closure, otherwise operates as normal.
	if ($disableLimiting) return false;
});

// Or do something after list manipulation takes place, like so...
$limiter->onBeforeManipulate(function(GridField $grid, SS_List $list) {
	SS_Log::log(print_r($list->map()), SS_Log::DEBUG);
});

// Setup a new relation editor.
$gridConfig = GridFieldConfig_RelationEditor::create();

// Since GridFieldLimitItems is not yet compatible with GridFieldPaginator, remove that now.
$gridConfig->removeComponentsByType("GridFieldPaginator");
$gridConfig->removeComponentsByType("GridFieldPageCount");

// Now we can add our GridFieldLimitItems component.
$limiter = new GridFieldLimitItems(10); // Limit to max of 10.
$gridConfig->addComponent($limiter);

// ... continue below with adding your new GridField instance with this $gridConfig...