Hard one this, definately dont go too large for thge CAB files. Between 10 and 20Mb is fine.
With regards age etc it would depend on a few factors such as how is your backup performing, what is your archiving policy in terms of date range? If it is taking an inconvenient amount of time for backups then bring collections closer to the archiving date so more is collected and your backup times go down. If you find you are not unhappy with your backups and in fact think they could actually run quite happily for a few more hours then decrease the time that data is collected to CABs.
Therefore if that makes sense there is no real best practise, just what works better for you and your environment.
