Quick take
Current SpringWell tiers use roughly 9-inch, 10-inch and 13-inch tanks for 9, 12 and 20 GPM service. RKIN similarly scales tank width from 8/9/10 to 13 inches as service flow rises from 9 to 28 GPM.
9-inch-class tanks
SpringWell CF1 uses a 9-inch tank at 9 GPM service. RKIN CS9 uses a 9-inch tank at 11 GPM service.
10-inch-class tanks
SpringWell CF4 uses a 10-inch tank at 12 GPM. RKIN CS10 uses a 10-inch tank at 15 GPM.
13-inch tanks
SpringWell CF+ and RKIN CS13 move to 13-inch tanks for much larger homes and higher service-flow requirements.
Why diameter is not enough
Valve design, media type, bed depth and connection size also affect usable flow. Use tank diameter as context, not the sole sizing rule.