The term `plan horizon` refers to the time frame or duration of a plan. It represents the period over which a plan is expected to be implemented and achieve its objectives. Understanding the plan horizon is essential for effective planning and decision-making, as it helps to set realistic goals and allocate resources accordingly.

Option 1, aggregate size of plan investment, is not correct. While the size of plan investment is important, it does not define the plan horizon. The plan horizon focuses on the time aspect rather than the monetary value.

Option 3, total plan outlay, is also incorrect. The total plan outlay refers to the overall budget or expenditure of the plan, but it does not specifically indicate the time frame or duration.

Option 4, growth rate target, is unrelated to the term `plan horizon` and is not the correct answer.

In conclusion, the correct answer is option 2, which correctly identifies the term `plan horizon` as the time frame of the plan.