The big ones (for furniture) look silly.
The smaller ones in the purple bag from HD fit the bill just fine for machines.
I want to say it was a 8 or 12 pack.
Edit: 16 pack!

IMG_3174 (resized).JPG

2-3/4" can still sink into the carpet like a regular foot would. Looks great I'm sure but, probably not the best choice.
Mine are somehing like 3-1/2" edge to edge. Sinks in juuuuust enough, but doesn't deform the carpet or pad if you decide to move the game around. (See pic above...)

I think he wanted to work on his WHOLE basement slide saves TheLaw. lol

Found them.
Everbilt 16 pack for under $9.

http://homedepot.com/p/Everbilt-3-1-2-in-Round-Reusable-Slider-Value-Pack-16-Piece-4743444EB/203672262

Have them on all my machines.
No complaints; easy to nudge/slap save.
Doesn't feel like the games are going to tip over...yes, even while death saving.
I guess the main key to that is to let the games sink into the foam first and then level them.
I usually give them a few days to acclimate under the machine before re-adjusting the pitch and level.