XNA Framework Reach profile
You are using the XNA Reach profile. This is a profile designed to make your game run on as many computers as possible. It is a pared-down subset of XNA's full functionality. The alternative is the HiDef profile, which exposes all of XNA's functionality, but has stricter hardware requirements.
requires mipmapped Texture2D sizes to be powers of two
One of the requirements imposed by the Reach profile is that textures with mipmaps have to have dimensions that are powers of two.
but this Texture2D is 800x800.
Your model is using a texture. The texture is 800x800 and has mipmaps (as is usually the case). 800 is not a power of two. The closest acceptable size would be 1024 (2 to the 10th power).
Resize it to a power of two, or remove the mipmaps.
Do one of these two things.