Jaemin :
Jaemin would be shocked hearing this but he'd be quick to make you laugh again and would constantly tell you he loved you for who you are.
Jeno :
He'd be upset that you were feeling this way about something he found so cute about you. He wouldn't really know what to do to make you not insecure but he'd always be there to cheer you up.
Jisung :
Jisung would be so lost about what to say but his jumbled words would be trying to say "but I love that about you". He wouldn't be one to bring this up again but at night he might wonder how you could be insecure about that.
Mark :
After you tell him this he'd want to know more like why you felt this way because he never saw anyone be mean to you about it. He'd then talk to you about why he loved your height trying to get you to give him a smile.