I can't imagine playing a warrior and trying to use the mouse to do my stance dancing... I would strongly recommend key binding the stances and doing it that way. However, I do understand that many people are just not comfortable using key binds.
The stance bar is probably the one bar you can most get away with changing the size of since it is not surrounded by other bars and game elements to the degree the other bars are. Just keep in mind that making it too large will start to overlap HUD elements, etc.
If you want to increase the size of that bar, you would edit [ Interface > AddOns > nUI > Layouts > Default > nUI_Layout_Default_Dashboard.lua ] with a plain text editor like "notepad" and locate the nUI_DefaultConfig.SpecialBars section then change the value of the btn_size attribute. Making the value larger will make the buttons larger. Just change the value, save the file then do a '/nui rl' to reload the page and see if the new size "works" for you or not.
Keep in mind that doing this changes the bar size for *all* of your toons. So... if you have a DK with a pet, or a lock with metamorphosis, then you can start getting some very large special bars.
What people don't get is that I am, ultimately, an artist at heart. My brush has two colors, 1 and 0, and my canvas is made of silicon.