Scale verts along 2 axis not 3


How do I scale these verts along the screen’s X and Y axis in this pic? I do not want them to change in the screen’s Z axis. I cannot hit S for scale then Shift Z to prevent a change in the Z direction since they are not aligned with the main XYZ axis as shown by the indicator in the lower left corner. I want to change the diameter of the circle, not the height. I hope this makes sense.
Thanks

Switch the transform orientation from ‘Global’ to ‘View’ and then exclude that transform orientation’s Z axis. In 2.79 terms, that would be S for scale then Shift+Z twice to exclude the axis of a chosen transform orientation. In 2.8x that would only be one Shift+Z.