How can I can create a named selection of spheres of certain volume?

Probably many ways of doing this, but below is one sample script that does that (modify as needed) - minimum volume and maximum needs to be adjusted to capture the sphere size needed.

mys=[]
mye=[]
vmin= 4.e-06 # min. volume of spheres in m3
vmax = 4.4e-06 # max. volume of spheres in m3
assembly = GetRootPart() 
bodies=assembly.Bodies
 
for body in bodies:
    if body.GetMaster().Shape.IsClosed==True: # check it is a solid body and not surface body
        s=body.Shape
        vol=s.Volume
        if vol >=vmin and vol <=vmax:
            print('sphere found : ' + str(body.GetName()))
            mys.append(body)
        else:
            mye.append(body)
            print('sphere not found : ' + str(body.GetName()))
 
 
 
 
primarySelection = Selection.Create(mys)
secondarySelection = Selection.Empty()
resultmys = NamedSelection.Create(primarySelection, secondarySelection)
NamedSelection.Rename("Group1", "Spheres")
Selection.Clear()
primarySelection = Selection.Create(mye)
secondarySelection = Selection.Empty()
resultmye = NamedSelection.Create(primarySelection, secondarySelection) 
NamedSelection.Rename("Group1", "Rest")

One can also look at how many faces the parts have (say a sphere might have only one and that can be then used as a criteria to find all spheres)