You need to make a custom rule to achieve this. Right now it's not possible to check against multiple dimensions as far as I know.
An alternative is using the min_height
'uploadImage' => 'dimensions:max_width=300,max_height=200,
Documentation: https://laravel.com/docs/7.x/validation#rule-dimensions
I would personally create a custom rule that supports this syntax: https://laravel.com/docs/7.x/validation#custom-validation-rules