suspect something your theme is doing, as works fine on my test site.
try
.bbp-topic-form, .bbp-reply-form, .bbp-topic-tag-form {
	width: 80% !important;
}
or
.bbp-topic-form, .bbp-reply-form, .bbp-topic-tag-form {
	width: 300px !important;
}
and play with the 80% or 300px – you can’t have both rules – they are just different ways of changing width.
		
	 
	
	
	
 
		
			
	
	
		
		Thanks but neither of those changed the width. I tried a variety of pixel and percentage options.
		
	 
	
	
	
 
		
			
	
	
		
		ok, not sure I can help further without a link to a live example under your theme.
		
	 
	
	
	
 
		
			
	
	
		
		Thanks, here is a link that should show it on mobile https://metpsy.com/forums/topic/welcome/.
		
	 
	
	
	
 
		
			
	
	
		
		you will need to open your site to anonymous posting to let me see the form
dashboard>settings>forums>anonymous
you can shut it down again afterwards
		
	 
	
	
	
 
		
			
	
	
		
		try
#bbpress-forums p input {
	width: 240px !important;
}
		
	 
	
	
	
 
		
			
	
	
		
		Bravo, you’ve done it again! Thanks.