diff --git a/telethon/client/uploads.py b/telethon/client/uploads.py
index 939718a3..919b1c31 100644
--- a/telethon/client/uploads.py
+++ b/telethon/client/uploads.py
@@ -201,9 +201,14 @@ class UploadMethods:
                 Optional JPEG thumbnail (for documents). **Telegram will
                 ignore this parameter** unless you pass a ``.jpg`` file!
 
-                The file must also be small in dimensions and in-disk size.
-                Successful thumbnails were files below 20kb and 200x200px.
-                Width/height and dimensions/size ratios may be important.
+                The file must also be small in dimensions and in disk size.
+                Successful thumbnails were files below 20kB and 320x320px.
+                For Telegram to accept a thumbnail, you must provide the
+                file's dimensions using either ``hachoir`` or
+                :tl:`DocumentAttributesVideo`.
+                Telethon will resize thumbnails for you, if an image is
+                provided.
+                
 
             allow_cache (`bool`, optional):
                 This parameter currently does nothing, but is kept for