This chapter addresses size estimating. It starts with a brief description of conceptual design and then covers proxy-based estimating. Chapter 6 describes the PROBE method for using proxies to make size and time estimates. As shown in Figure 4.1 in Chapter 4 (see p. 63), the PSP planning process starts with the conceptual design. This provides the basis for the size and resource estimates. At the outset, you will know relatively little about the planned product, so your estimates will be least accurate. Because you must presumably commit to a delivery date, however, you must make the best estimate that you can. This chapter describes how to identify the parts of a planned product and estimate their sizes.