Index: org/openstreetmap/josm/data/validation/tests/TagChecker.java
===================================================================
--- org/openstreetmap/josm/data/validation/tests/TagChecker.java	(Revision 13413)
+++ org/openstreetmap/josm/data/validation/tests/TagChecker.java	(Arbeitskopie)
@@ -448,7 +448,7 @@
             }
             if (checkValues && (value != null && value.length() > 255) && !withErrors.contains(p, "LV")) {
                 errors.add(TestError.builder(this, Severity.ERROR, LONG_VALUE)
-                        .message(tr("Tag value longer than allowed"), s, key)
+                        .message(tr("Tag value longer than 255 characters ({0} characters).", value.length()), s, key)
                         .primitives(p)
                         .build());
                 withErrors.put(p, "LV");
@@ -455,7 +455,7 @@
             }
             if (checkKeys && (key != null && key.length() > 255) && !withErrors.contains(p, "LK")) {
                 errors.add(TestError.builder(this, Severity.ERROR, LONG_KEY)
-                        .message(tr("Tag key longer than allowed"), s, key)
+                        .message(tr("Tag key longer than 255 characters ({0} characters).", key.length()), s, key)
                         .primitives(p)
                         .build());
                 withErrors.put(p, "LK");