OK, here's a spreadsheet that I cobbled together to show the various parameters of a Classical Cassegrain. Hopefully, I've entered the formulas correctly (rows I thru J are from the Wikipedia page for Cassegrains and L thru N for calculating field curvature from telescope-optics.net; both links are in the spreadsheet).
As you can see, as the primary becomes slower, the field becomes flatter but the secondary conic increases which presumably make it harder to make. As the primary becomes slower, the minimum secondary size increases as well.
You can change the numbers in the highlighted fields for different primary-size/system f-ratio/back-focus combinations to see how everything else changes.
Note: click the 'edit workbook' button to enable editing of the spreadsheet.