The objective of creating a silhouette using flash is to throw an orb of light behind your subject and then underexpose your subject against that light. But the shape, size, and color of that orb of light can vary drastically depending on many factors. I am going to go through all those factors so you can understand how they affect your light.
Distance
The distance between the light and the background has an impact on the size of the orb. The closer the light is to the background, the smaller the light will appear. Conversely, the further back the light is from the wall, the bigger the light will appear.
Flash Zoom
The amount that the flash is either zoomed in or zoomed out will also have an impact on the size of your light. The more you zoom in, the tighter the light will become. The zoom level will also impact the quality of the light. Often times if you are zoomed all the way out, the flash will cast a shadow of the Fresnel lens. For that reason, I am usually at about 50mm on the zoom level, and then I use distance to size the light exactly how I want it.
Using light position to shape the light
The shape of the light can be determined by two factors: either the modifier you are using or the position of the light and how it falls onto the background. If your light is low and pointed high onto your background, you will get an oval shape. The same is true if you have the flash either to the right or left of the center of your subject and then have the flash aimed towards the center. That will also create an oval shape as light will hit the background at an angle.
Using a modifier to shape the light
The shape of the light can also be determined by the modifier you are using. Bare flash will often create a bit of an oval shape because the flash head is rectangular and throws off a rectangular shape that defuses into an oval. Putting a MagGrid onto the flash will shape the light into a more circular shape. Putting two grids on the flash will make it even more circular with sharp edges. Using the MagSphere will spread the light out, making it a more organic and defused shape. You can even get extra crazy by using the MagBeam to throw a pattern onto the background.