Where to begin... (General)
Could you tell me where/how to change something such as the size of the input field on the 'New topic'/create post page?
This is a task for the HTML sources. You can change the size of the input fields in the respective templates. For the posting form this is templates/[theme-name]/subtemplates/posting.inc.tpl, beginning with line 54 (in version 20220803.1). Please do not remove or rename any of the input fields, even you don't see them in the browser. What you can change without causing any issues are the values of the attributes named
Trenne niemals Müll, denn er hat nur eine Silbe!