Edit this file /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css
Look for this line (around line 1200): -GtkRange-slider-width: 10;
And change 10 to a bigger number like 20 or 30, and done :)
I also think they're too small...
See this image:
If you change the scrollbar size to 16 and you want them to stay that way, and not shrink when you don't have the mouse on it, then make the changes in the picture above.
If you use a size other than 16, just experiment with the margin sizes to get it right.
Best Answer
You can disable the scrollbar completely (still able to scroll with mousewheel) from the Terminal Preferences.
Let me know if this helps.