The maximum radius appears to be limited to the size of the largest visible object in the scene.
I think for some tools a larger radius might be preferable. I like to use a huge move radius for example to get some interesting initial shapes from a sphere, I believe a lot of sculptors like to work like that.
My suggestion is to allow overshooting the radius to 200%, maybe 300% of the largest visible object, similar to the power setting, where 100% is the maximum indicated setting, but more is allowed by moving the slider further up.
update: turned out a larger radius is possible with the “screen” radius setting instead of “constant (3D)” radius setting which I apparently had switched at some point 
… Still leaving this suggestion here as a possible addition when the “constant (3D)” setting is enabled.
when using a setting outside the range maybe this could be indicated by a overlay:
