diff --git a/CHANGELOG b/CHANGELOG
index 8623edc577..980a0c8c29 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,3 +1,4 @@
+* Add support for size on Many2Many
 * Allow to show plain text in password field
 
 Version 4.2.0 - 2016-11-28
diff --git a/src/view/form.js b/src/view/form.js
index 01236509cd..6e982b5573 100644
--- a/src/view/form.js
+++ b/src/view/form.js
@@ -2957,9 +2957,15 @@ function eval_pyson(value){
                     this._required);
         },
         _set_button_sensitive: function() {
-            var size_limit = false;
-            if (this.record() && this.field()) {
-                // TODO
+            var size_limit = false,
+                record = this.record(),
+                field = this.field();
+            if (record && field) {
+                var field_size = record.expr_eval(this.attributes.size);
+                m2m_size = field.get_eval(record).length;
+                size_limit = (((field_size !== undefined) &&
+                            (field_size !== null)) &&
+                        (m2m_size >= field_size >= 0));
             }
 
             this.entry.prop('disabled', this._readonly);