Hi Barracuda
It is very simple. Select your geometry. click the select tool, and select the green box on the edge of the yellow frame [See your image] and drag in the direct you want to change the diameter in. You may have to use shift or control to scale about center, Then type say 30mm. it is important that you type the radius size with the “mm” otherwise it will scale it as a ratio.
Very few sketchup users know this.
As a point of clarification, using the Scale tool on the geometry shown selected in the OP’s original screen shot will deform the transition areas at the ends so it is not the right option for the task.
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With this method ,it is important to know that you’re rescaling the DIAGONAL to a specific size. In order to resize the diameter of a circle , you would have to scale it twice to a specific diameter 
But as @DaveR DaveR mentioned , You would also have to select ‘Planar Vertices’ so the scale grips are all in one plane (2Dimensional), for it would scale deformed if the selection contained vertices on more than one plane.
I agree with @Box,that @Wo3Dan’s solution of redrawing the profile is far more efficient.
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I know it’s a paid extension, but Thomthom’s Vertex Tools does a “soft select radius” that will allow you to scale the geometry in the center without breaking the curve at the ends:
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