I think the issue is related to the CRS of your file. When you use Polygon Divider plugin you need to make sure that the size you choose is appropriate using the projection of the polygon.

For, example, I have polygon shapefile with a UTM Projection:

[![enter image description here][1]][1]

I put 10000 km2 as the desired size in CRS unit which UTM:

[![enter image description here][2]][2]

Here is the output with attribute showing the Area:

[![enter image description here][3]][3]

Make sure you are not using geographic spatial reference (WGS84) or other any other projection that is not in meter unit. If you are in geographic CRS, you need to reproject your polygon layer with appropriate CRS of meter unit.

  [1]: https://i.sstatic.net/OaVnK.png
  [2]: https://i.sstatic.net/x5Jl2.png
  [3]: https://i.sstatic.net/7C4Rl.png