The cursor splat is defined in the ability's field Effect - Cursor Effect. For Psi Storm it's set to the effect High Templar - Psi Storm (Search), and by increasing the radius of this effect, the cursor effect will be bigger.
Have the upgrade increase the size of the effect your cursor effect uses.
(I have never tried upgrading a cursor splat like this, so I'm sorry if it doesn't work as I think)
The cursor splat is defined in the ability's field Effect - Cursor Effect. For Psi Storm it's set to the effect High Templar - Psi Storm (Search), and by increasing the radius of this effect, the cursor effect will be bigger.
Have the upgrade increase the size of the effect your cursor effect uses.
(I have never tried upgrading a cursor splat like this, so I'm sorry if it doesn't work as I think)