How can I made my arrows bigger?

At your  arrows if you turn on the expert mode, you will see the "Responsive" options, where 1 is the original size of the arrow, so you could increase that number:

Free

If you are using the free version, use this code instead, and put it into one of the css files of your theme/template, where the slider loads:

.nextend-arrow{
	width:100px!important;
	height:100px!important;
}