The first is the Options editor, which is generally unimportant. At this point there are a couple of selections, and to understand them we need to consider the concept of partitions. Unix has the concept of partitions too, but they are quite different from their similarly named MS-DOS cousins. In other words, the majority of DOS files are routines to write and manipulate data on the disk. The Unix OS, by contrast, is concerned with a great many other things than disk manipulation.
Bigger disks have more blocks, smaller disks have fewer.
Under DOS, there are primary partitions, extended partitions, logical partitions, etc. While disks can be split into many multiple partitions or a single partition, a Unix system must have at least one swap and one filesystem partition. So there are two choices in the Allocate Disk Space for FreeBSD menu item. The second attempts to figure out what DOS is using for the number of heads, tracks, sectors and will attempt to allocate the Unix partitions so as not to stomp on any other operating system resident on the disk. After allocating the disk space, go into the Label Allocated Disk partitions.