I'm a lefty, too, actually,... what I do is operate the mouse in my left hand and I've rebound all of my action bar keys to the number pad on the right side of the keyboard... I smoke 'em now ~smile~
That said, it is possible to increase the size of the pet bar... edit [ Interface > AddOns > nUI > Layouts > Default > nUI_Layout_Default_Dashboad.lua ] with any plain text editor and look for the nUI_DefaultConfig.SpecialBars section. You can change the value of "btn_size" to make the buttons bigger. However, it will likely have some side effects since it will make the entire bar larger, etc.
Odds are that once you get used to it you'll come to appreciate the smaller button size... it means less mouse travel between buttons among other things.
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.