Here's a quick validation rules for image dimension in Laravel

public function postImage(Request $request)
    {
        $this->validate($request, [
             'avatar' => 'dimensions:min_width=250,min_height=500'
        ]);

        // or... 

        $this->validate($request, [
             'avatar' => 'dimensions:min_width=500,max_width=1500'
        ]);

        // or...

        $this->validate($request, [
             'avatar' => 'dimensions:width=100,height=100'
        ]);

        // or...

        // Ensures that the width of the image is 1.5x the height
        $this->validate($request, [
             'avatar' => 'dimensions:ratio=3/2'
        ]);
    }

Happy Coding !