Size validator for comment dimensions and margins

src/PhpSpreadsheet/Helper/Size.php

@@ -26,12 +26,12 @@ class Size
         return $this->valid;
     }
 
-    public function size()
+    public function size(): string
     {
         return $this->size;
     }
 
-    public function unit()
+    public function unit(): string
     {
         return $this->unit;
     }