when I bought my marin back in 1992 ISTR it was road less 3-4 inches hence I ended up with 19" frame Marin and ride 22.5-23" road but the marin does feel bit big and prefer a 17.5" in same frame now, the issue is less the seat tube as can adjust seat pin but rather the overall reach and position,
I think it rather depends on make and really endorse the comments above, it's just not that simple.......unfortunately