If you forget - as I do every so often - it defaults to... 128 bytes.
Why 128 bytes rather than one?
Because that was the size of a sector on the original 8" floppy disk, which had 77 tracks of 26 sectors back in the early 1970s. Early versions of CP/M could only read and write in 128 byte chunks, and Turbo Pascal was originally released for CP/M, so that was the sensible value for the default.