For installation and a lot of OZ administration, it's crucial to understand the overall memory layout on the Z, how OZ works with it and how you can manage that memory by later managing filesystems and moving files and directories around.
In terms of storage, the 5000D and 5500 have:
16M of flash memory for the operating system itself,
RAM (32M on the 5000D and 64M on the 5500) which can be used for both OS memory and extra file and directory storage.
In addition to the above, you can add extra memory/storage with the use of either of:
the CF slot which can hold, among other devices, a CF memory card, and
the SD/MMC slot, which can be used for even more storage.
All of the above will affect which image you choose to install and how you make use of your memory - more on this shortly.
Unlike the original Sharp ROM, the OZ image treats the 16M of flash as writable which means that, even after initial installation, you can continue to make changes to the files and directories in flash, including installing new software to the directories in flash. It's important to understand this since, by default, if you start installing new software, those files will be installed in the few remaining megabytes of free space still in flash.
Putting it another way, even if you install and allocate extra storage in RAM, or have even more storage on a CF or SD card, you have to explicitly state that you want to install to those locations - all of this will be discussed in more detail later.