Actually, as I understand, current widget box expands automatically but cannot shrink yet. When you restarted widget it began from small box with "update", and later expanded a bit to fit regexp match. There's absolutely nothing in my patch that could fix it.
So it'll keep a small size until you start experiment with regexp again.

I may be wrong as I never worked with GTK (for this patch I just copypasted existing GUI code. I hope KiberGus will find some time to add autoshrinking for widget.