Hi Cosari & welcome to TalkGraphics.

You seem to have two issues.
1: To determine the animation file size
2: To offset/reposition/reduce the animation size.

So first & foremost I wouldn't include your background image within an animation. 1920 x 1080 is a very large size for a gif animation. Further gif animations can only render at 256 colours.

My way to tackle this would be to open a web-page in Xara and import your 1920 x 1080 image.

Next within Xara 3D Maker I'd create a far smaller transparent animation, say 500px x 300px. Xara 3D maker can create some aweful jaggy edged transparent animations. Best to match the background colour to the one it's going to be positioned over.

Import this animation into the web-page created above. Move & resize it to the required position.

Publish the page.

The resultant page has a background webp image size of 225Kb and an animated gif of 455Kb.
700Kb compared to a 17,000Kb at the best you can create alone within Xara 3D Maker.

DEMO