@Richardus.Raymaker, what you are seeing is another one of those broken UI problems with edit.js first, a light source is always spherical, so all the dimensions must be the same. Using the default and broken editor, you can increase any of the X/Y/Z dimensions, either using the up arrow or typing in a larger value, on a light source and it will set the other dimensions to the new value. However, decreasing the values has broken logic and edit refuses to make it smaller.
This is one of those priority -83753 issues so I wouldn’t make plans. For now the easiest way to work with it is to click on the “Reset Dimensions” button. That will set the light to 1m. Then you can enter any value greater than 1m. The editing corners work too though they tend to be twitchy.
For spotlighting, there are options to do that. Scroll to the bottom of the entity editor. There you can set falloff and angles.