I've heard it said that older rods rated at say, 6 wt would be best paired with a modern 5 wt line. For example, I have an original Sage 6wt/7wt 9' rod (no model designation), would that best be matched with a 5 wt / 6 wt line? Is there a similar "rule" for reels? I could go through trial and error but figured I'm hoping there is some common knowledge about old rods and new lines.
Back in the day, floating weight forward lines rarely had head lengths over 30 feet. Today's weight forward tapers have lengths well over 30 feet. Therefore you are gaining more line mass outside the tip of the rod like a double taper. Remember that a double taper has a consistent belly until the taper on the back end. So if you have a modern 6 weight weight forward taper on a 6 weight rod, it might feel like you are throwing a 7 weight line. Drop to a modern 5 weight line, that rod will feel like a 6 weight again. This is a practice that some people do with older glass and bamboo rods.
As for reel size, keep in mind that large arbor reels aren't that old. Abel did not have a large arbor design until 1999. The old school Pflueger Medalist 1492 was rated for up to a 5 weight line without backing. It has a spool diameter of 2 3/16". My Abel Creek 2 LA that I use on my 5 weight has a spool diameter of 3 1/2", and it will accomodate my 95' long WF5F line and 75 yards of 20 pound backing.
By today's standards, look at the spool diameters for large arbor reels. 0-3 weight, 2.75" - 3" 4 weight, 3" - 3.25" 5 weight, 3.25" - 3.5" 6 weight, 3.5" - 3.75"
There is overlap between rod weight categories and spool diameters. I did not mention the width of the spool. That is a factor on capacity. Due to CNC technology, reels are being slimmed down. They are lighter. I use 3.5" reel on my 9 foot 4 weight because it is light enough to balance it.