For your application in a 72-inch diameter, 120-inch straight-side tank, you should be able to fill it with about 2,000 gallons of a 1.45 specific gravity, 6.2 cp liquid.  Mixing with three 27.5 inch diameter pitched-blade turbines, the shaft length looks reasonable and the off-bottom locations for the impellers are helpful.  The shaft diameter of 3.5 inches is larger than necessary (1.5 inches), but the extra size is no problem in this case.
 
The mixer should give you plenty of mixing intensity if it operates at 68 rpm, which will require a 5 hp motor.  For somewhat less intensity, 3 hp at 56 rpm should work.  If you think you need more intense mixing, then 7.5 hp at 84 rpm would be required.

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