I understand your concern but having pushed over dozens of trees in the 8"-18" truck size diameter I can say that I've never come close to having the root ball hit my tractor. For starters the root ball is rarely more than three or four feet wider than the trunk on each side (usually only a couple of feet in my experience) and the FEL bucket is at least that far in front of the tractor nose. Secondly, you push the tree over until it starts to move on it's own and it usually takes a good five or ten seconds to slowly topple which is plenty of time to back away a few extra feet.
Yeah, I've done it more than a few times myself, but always disliked being so close to the tree with the tractor due to widowmakers above and the unexpected below. The actual root ball can be avoided but I've seen them raise some really big lateral roots as the tree fell that would be trouble if they caught the tractor. It's something to be aware of that a novice operator may not anticipate. With tree cutting, you have to expect the unexpected, and even then, stuff happens! Take care.