I am trying to make the navigation arrows larger.
The size of the navigation buttons can be made larger by increasing the value of the navButtonIconSize configuration option (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section). (There is no need to change the icons.)
With regard to your problem with the 'juicebox.dev.svg.zip' file, I have just downloaded the file myself and it is OK.
If you would like to check the integrity of the file you have downloaded, it should have the following properties:
Filename: juicebox.dev.svg.zip
File size: 4,820 bytes
SHA1: 212a127fd00ac840fd1e1f7108255a8de12b5f07
You can check the file's SHA1 hash with a program such as HashCalc (free).
If the file you have downloaded does not have the properties above, then please try downloading the file using a different browser (Firefox, Chrome, Safari, Internet Explorer, Chrome).
If the file you have downloaded does have the properties above but you are having trouble extracting the file, then please try a different unzipping program such as 7-Zip (free).