USB drives aren't quite like SD cards where the sizes matter. As SD cards expanded, they needed more address space and had to refine the standard for larger cards. USB easily handles terrabyte drives.
So SkyView doesn't care how big the drive is. It does however only support FAT and FAT32 drive formats. As long as you format your drive FAT32, it will work.
Like bret said though, 128GB or even 64GB is overkill for what you'll use on SkyView. Most people use 16GB drives, which is also what we ship with the units. 16GB will easily hold your charts, datalogs, updates, and more: