I agree. The actual size of the sphere of exploration will be significantly influenced by two things, IMO.
The first is travel time. If you are dealing with manned expeditions, you are going to have crews that are going to have to be prepared to undertake very long missions. There is usually going to be a limit to this.
The second is volume. The further out you travel, the bigger the sphere you are exploring becomes. You can quite easily run into "space is big" problems. In these cases, you may end up in a situation where only a relatively small sphere is "fully explored", and a much larger volume has been only partly explored.
