It is not possible to change the size of the navigation hit areas but if you set imageClickMode="NONE" rather than the default value of "NAVIGATE" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section), then the user will need to click specifically on the navigation arrows (and clicking elsewhere on the main image will do nothing).
On touch-enabled mobile devices, users will still be able to swipe to navigate between images.

You can change the size of the navigation arrows via navButtonIconSize (also in JuiceboxBuilder-Pro's 'Customize -> Main Image' section).

Also, if you want the navigation arrows to be displayed on mobile devices as well as desktop browsers, set showImageNav="ALWAYS".

Short descriptions of all the configuration options in bold above can be found in the Main Image section of the Config Options page.