Can I adjust the size and shape of the watermark on Gumlet?

Yes, in order to adjust the size of the watermark, follow the given tutorial to adjust the watermark size.

Note: You can not adjust the shape of the watermark.

From our Dashboard

• Click Profile under the View drop-down menu.

• Select the profile you want to edit.

• Click Overlay & Padding Options > Image Overlay.

• Insert the image URL.
• Not only that, you can adjust the dimensions too.

From API

• Use this API to do so and add the image URL under the IMAGE_OVERLAY OBJECT section.

Let me know if you have any queries.
If you wish to add thumbnails to your video then visit this thread: How to insert thumbnail?

Or you can read this article: What is Video Overlay?