Eloquent cannot find Code Igniter Model

[eluser]coderego[/eluser]
So I am using Eloquent with code igniter and getting some interesting bugs.

Code:

class Brand_model extends MY_Model {


public function size()
{
    return $this->hasOne('Size');
}
}

That line errors when trying to load my Size model:

Code:

class Size extends  MY_Model {

    public function brand(){
        return $this->belongsTo('Brand');

    }


}

"Unable to find class Size"

Any ideas?