Relative size. They look a little smaller here, but they are about medium size.
The tabs on the side hold the button in the hole that you drill for it. They compress and push out and there are multiple "steps" at the top depending on the thickness of the mounting material.
The inside. The top cover is pretty easily taken off by pushing in the snaps/tabs. The white part extends down to press on the switch on the bottom. It has a large white top surface, which makes the tops of the buttons look whitish and not see-though.
The white top part attached with no top cover.
Inside. There's a pretty good amount of room to mount lights, a little bit of circuitry, etc.
Here's a side view so you can see how much room there is. It's translucent, so you can see the center white post through the button and see how much room there is on the side of it.
The button, which is released by tabs in a similar way to the button top.
Putting an LED in. There is plenty enough room. I'm using an LED with long leads that can be bent around and through the side holes. It happened to be a tall, smooth LED with no rim on the bottom. It's green, sorry that it blends in with the blue switch.
I would suggest putting two in on each side of the switch, because the light gets diffused some by the white top, so it partially looks like a dot of color on one side. Putting two on each side would make it much better. The light goes through the button alright (like fiber optics), but again it would be better looking with two LEDs.
Side view of an undepressed button. The top is slightly convex, which is a little hard to see on camera, but easier in person. It's hard to feel, the top feels pretty flat. The datasheet also shows that it is convex.