The best way to get something close to a sphere with the Glass filter is to do the following.
1 - Start with a circular selection. It is best for you to make a note of the radius of the circle. You will use it in step 4!
2 - Start the Glass filter.
3 - Go to the Bevel Profile tab. Under Factory Profile, choose "Button" which is the default value anyway.
4 - Go to the Basic tab and set Bevel Width to the radius of your circular selection. A lower value flattens the button while a higher value will create a sharp point in the middle, sort of like a Hershey's Kiss. Fiddling with Bevel Width is the most important step.