as usual, the answer is "it depends".
In theory, there should be no issues as long as your server bandwidth can process the archives in reasonable time. However, as rule of thumb, I'd suggest to stick to 300-500Mb at maximum. Going after that, you could incur in some limits imposed by hosting companies. For example, some hosts will kill the process if it's creating too large files, if you get past 2Gb you'll get into PHP "dangerous" area. Recent versions of PHP are usually compiled under 64-bit architecture, but if you get an older version, it could be compiled for 32-bit.
This means that there's an hard limit on how an integer number can grow. Akeeba Backup will force the creation of a new backup archive once it's reaching such limit, but in some cases the PHP process can start acting weird while handling such large numbers, so it's better to avoid that.
Finally, the same applies to your bandwidth. Maybe right now it's fast enough to process 2Gb in one shot, but in the future your server could get more sites and the transfer speed could drop, resulting in timeouts during the upload.
So I'd suggest to stick with a part size of 500Mb , it will lower the chances you'll have to tweak your configuration in the future.
Developer and Support Staff