Quick take
Measure hardness first. A family that looks like a 32K buyer at 5–8 gpg can become a 48K or 64K buyer at 20–25+ gpg without changing household size.
5–8 gpg: moderately hard
Capacity is consumed relatively slowly. Smaller households often fit 24K–32K systems, while larger families may still fit 32K–48K.
10–15 gpg: hard water
This range often pushes typical families toward 48K-class systems because daily grain load rises quickly.
20–25 gpg: very hard water
At this level, larger resin capacity becomes much more useful, especially for four or more people.
25+ gpg: test and calculate carefully
Very hard water can justify 64K+ systems, but service flow, iron, manganese and actual daily gallons should be included before ordering.