There is no one right answer to question #1. On a 32 bit server there is a hard limit of 2 Gb per file. There is a similar limit for 64 bit servers, but I can't count that high. If you are moving your archive to cloud storage, some of the cloud storage services have maximum file size limits. Some hosts also have maximum file size limits. If you don't have any of these limitations to deal with, you can set the part size for 2 Gb (you have to set it in megabytes) and it will be a long time before it splits.
The Joomla! central update center is used by many programs, including JCE and Akeeba products. Some extensions do not use it.
On the Akeeba Backup screen, on the right side, click the black "Reload update information" button. That will make the update notice go away. Joomla! caches the update information and doesn't update the cache when you update the extensions. Some folks consider that a bug, others will simply tell you it is supposed to work that way. I'll just leave it at that.
Dale L. Brackin
Support Specialist
English: native
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!
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