https://i.imgur.com/nEoxpfd.png
The average is 3.1mb per file.
When looking at my SavedVars folder, I feel like the files with more than 1mb are not important to be auto-saved.
The really large files (MM**.lua and ArkadiusTradeToolsSalesData**.lua) contain sales data from the guild history. If saving data of the current play session fails, the data is simply re-obtained upon the next login when scanning the guild's history. So I don't think these are files that would require auto-saving.
The HarvestMap**.lua files contain locations of crafting resources. If these files are large, then the player typically used some external tool to obtain data, in which case it is also not a big issue, if the data is lost.
However, if the files are small, then they typically only contain data collected by the player themself. In that case the player would be rather sad to lose the collected data from the current play session.
Personally, I think I would use the optional save request for HarvestMap to accommodate for these two different cases.