I think SketchUp should have the ability to enter circle specs as radius or diameter, and AK_SAM’s circumference and area would also be very useful.
A method for me is converting the desired diameter to an integer. E.g., 1-1/8" is 1.125", and convert the “2” divider by the same multiple, so I enter 1125/2000 to yield the 0.5625 radii.
Area is the property of a different entity, the enclosed face that might not even exsist.
Desired perimeter would be a good one for change. Though it would be a different length than the sum of all segments.
p.s. as long as the circle holds the metadata of the true perimeter the shown length is the perimeter, not the sum of the segments.
can of worms right there 
Not at all, just that you combined two separate entities in one entity window.
How and where would you make use of area?
With the ‘Tape Measure’ tool active you can still change the circle’s radius and see perimeter and hover over the changed face to inspect the changes in area.
I see your point in being able to change the diameter or the perimeter in ‘Entity Info’.
Area is the result they would have on a different entity.
I second that Dave 
My party trick when I was younger was multiplying 2 random 3-digit numbers in my head (try 973 x 567) to see how tough it is.
Nowadays I find myself grabbing a calculator for fairly straightforward stuff - but I always put it down - then pick it back up to check my answers 
In North America, pipes are usually available in inches.
Foe example a 4 in nominal diameter pipe has an outside diameter of 4 1/2 inches or 4.5 inches.
The radius is then 2.25 inches.
I would very much like to be able to enter 4.5d or 4 1/2d in the Measurement window.
I already submitted that feature many years ago.
Anyway, I use The Engineering Toolbox to get pipe fittings, including pipes themselves but I can understand the problem with any other circular object to drawn.
I think if one works frequently with pipe or hardware in North America then this request makes enormous sense. If you don’t then I guess it’s hard to understand. I also have requested and would love to see circles by diameter.
Can of worms - meaning how do you calculate circumference in SketchUp? PIr^2 or length of boundary edges?
Areas could be useful to some people…landscape architects, urban planners, etc.
The Circle tool creates a circle and fill at the same time (the icon even shows it as a circle with a fill), so I don’t think it would be wrong for the measurements field to report area while it’s being created.
Rectangles, should do this, too…I’d say that’s more useful to more people than a circle reporting its area.
I edited my previous post, the OD of a 4 inches pipe is 4.5".
If using the metric system, a 100 mm pipe has an OD of 114.3 mm which happens to be 4.5".
I find it easier to type 114.3d instead of 57.15 as a radius.
If SU’s programmers eventually implement this feature, I would like it to stay the default once changed to diameter. Adding r after a measurement would bring the default back to radiuses.
Of course, the label in front of the Measurement Window would indicate “Diameter” or “Radius” and the status bar would be consistent with that change.
How about making Dia/R a choice in preferences?
I totally agree.
or an alt tool thing. Just like you can draw a rectangle from its side or its centre. Drawing using dimension from the sides (diameter) or the centre (radius) just by pressing alt / option.
could be an elegant on/off solution.
Another way to skin the cat.