I created a button on the index.php to show "by categories", but the size
I created a button on the index.php to show by categories, but the size change when the window is around 825 width. You can see what I meant in the attachment below. I need it to keep the same size under of the 3 post of any specific category. Any help I will appreciate it. Thanks.
Please note that as this is a custom modification I can not resolve the issue just from looking at your screenshot. Could you provide URL to the affected page so I can have a look at the code and give you some tips?
Also, just a reminder here, please do not edit the theme files. Use a child theme instead. I am not sure where exactly you want the button to display (Only on your homepage? Where exactly and what should it do?) but you should use an action hook to display it. The theme is full of useful action and filter hooks. If you describe in more details what is your goal here, I can provide you with some code tips that could help.