I would like some guidance on how large to make the Archive file. We don't want to lose data, so I would think I should allocate it large enough to hold a full day (with some extra room to spare) and run a backup & reset of the Archive file every night.
Can you tell me on average how many transactions per cylinder (or similar metric) the Flashback and Archive files can hold? )
The number of transactions you can store depends on what you are monitoring and how many resources those transactions used, so there isn't even a rule of thumb that can be used for size.
You can check how full the Archive file is by selecting PF2 Summary from the explore screen to get some idea how full it is getting throughout the day. With this information you can tell if you have enough room to store at least one full day.
The size of the Flashback file will depend on how much data you would like available online.