Spatial query to get enclosing radius/polygon of result set
Yes, using SQL Server's


"Static Aggregate Geometry Methods"
"Static Aggregate Geography Methods"


You essentially have three options


ConvexHull: returns a polygon that essentially wraps the input with a rubber band,
EnvelopeAggregate: returns a bounding box (polygon) that encapsulates the input (box = lines in a cartesian project, arc segments in a 3d/spheroid/geographic projection)
UnionAggregate: takes all of the inputs and composes them into a single complex-polygon


Note: you mention radius, a radius is a buffer over an input in GIS.

If you need more features, you should check out the free PostgreSQL and PostGIS which supports even more GIS functionality.