Lombard supports a maximum drive height of 12.7mm, which means that 10 GB is its maximum capacity of drives that are currently shipping. The larger IBM 14 GB drive (available from MCE) is 17 mm tall and will only fit in Wallstreet. From page 40 of the Lombard Developer note:

Hard Disk Dimensions

Figure 3-2 shows the maximum dimensions of the hard disk and the location of the mounting holes. The hard disk is physically smaller than the ones in previous PowerBook computers: only 12.7 mm (0.5 inches) high. The minimum clearance between any conductive components on the drive and the bottom of the mounting envelope is 0.5 mm.