wpf richtextbox taing one character per line, when no width or minwidth is set

I have a richtextbox inside a grid and a stackpanel. The problem is I am not setting the width of richtextbox. I want it to occupy according to its parent. But when I type characters, its displaying one character per line.

If I set minwidth, thw window is getting scrollbar, because window is less than minwidth of richtextbox (which is not desired)

Can anybody help me out to resolve this problem
Thanks in advance