The flex method will not apply to the buttons. You need to set the button's dimensions using the native options.
Please note that our snippet serves as a guide or starting point only. Though it would make sense to use CSS to set the button's height, writing custom code is not a part of our support service. We could not continue on writing custom code to achieve specific needs moreover that there is a native option.
If you wish to continue with CSS, you will need to consult with a third party developer.
Thank you for understanding.