Would a slower and larger rotor blade be practical?
Indeed a relatively large rotor has advantages:


Low induced velocities: less sand and bushes that are swept up by the rotor airstream.
Low autorotational rate of descent.
Low power required in the hover.


However, there are some disadvantages as well:


The size of the helicopter is less practical at landing, take-off, clearance of buildings etc.
The empty weight is higher, a very important factor in VTOL aircraft.
The hub drag is higher.


A smaller rotor will have a smaller and lighter hub and a lower overall parasitic drag: more efficient for cruising flight. There is a practical limit to blade radius of about 12 m due to static rotor droop when not spinning. From J. Gordon Leishman:


  The static droop can increase quickly for larger rotor diameters and may cause problems when starting and stopping the rotor, especially in gusty wind conditions where the low centrifugal forces on the blades may lead to "blade sailing,"causing the rotor blades to flap and flex and perhaps impact the airframe.


The rotational speed of the rotor is determined by the tip speed, and is set such that at cruise speed the advancing blade tip stays below the drag divergence Mach number. Reducing the tip speed permits a higher flight speed to be achieved.

Image source

There is a design optimum for rotor disk size, determined by weight. The weight over disk area ratio is the disk loading and the analogy with wing loading of fixed wing aircraft is very strong: a large wing area creates more drag and construction weight. The relationship between disk loading and operating weight is partly governed by the square-cube law, and a clear trend can be observed when the two parameters are plotted against each other: there is an optimum governed by weight, and the optimum is not the largest rotor diameter.