Tom,
Flynn designed the 260 yard slightly uphill 10th at Rolling Green (follows the uphill 614 yard par 5 9th). It was always a par 3 on the scorecard. I'm certain that under firm and fast conditions, a low running draw made the green reachable for the better class of players in the mid-1920s. Clearly Flynn was requiring a very specific and high-demand shot to get a 3, however he probably wasn't considering par in of itself very much but creating a hole that made for interesting match play options where executing a high demand shot really rewarded the player who takes on the risk. If that is what was being designed in the 1920s, are there any modern designs of 300 yard par 3s? That would seem to be about the ballpark equivalent of 260 yard par 3s back in the olden days.
Flynn's desire for variety influenced him to design short, medium, and long par 3s, 4s, and 5s. Without taking topography into account, on the long side, Flynn designed par 3s between 240-260 yards, Par 4s between 430 and 460, and Par 5s between 570 and 620 yards. On the short side, Flynn designed 110 yard par 3s, 280 yard par 4s, and 460 yard par 5s. Flynn was surely influenced by, if not directly involved in Merion's nice blend of long and short par 4s.