As Spinebot mentioned, the player determines the animation size by "calculating the maximum boundary of the animation at runtime." This function is used for that purpose.
If you need precise sizing, it's better to pass the viewport coordinates and dimensions to the player, as described here.
If you want an animation to have a specific size—for example, 1920x1080—you can add a 1920x1080 (or larger, but still 16:9) rectangle placeholder to your Spine project. This placeholder won't be exported.
Place the rectangle strategically so that it's easier later to pass the correct viewport coordinates and size to the player.
Animators should keep their work within this visual guide. Then, simply pass the visual guide's dimensions to the player's viewport settings. If the placeholder is centered on the origin, your x and y values will be -width/2 and -height/2.
This discussion might be useful too.
