Here's the expansion module: https://www.synology.com/en-us/products/RX1217
Here's the list of compatible drives: https://www.synology.com/en-us/compatibility?searc...
Here's the documentation that tells you how to do it: https://global.download.synology.com/download/Docu...

LoPath
Maintain HiPath 4000 V5 & V6, OpenScape Xpert V4 & V6, OpenScape Xpressions V7, OpenScape Contact Center V8, OpenScape Voice V9

EAdams2009,
This will depend on what you want to do with your RAID set. If you want to expand the existing raid, you will need to match the expansion drive size to those in your existing array. (This is not specific to Synology, but RAID in general). It will particularly matter for RAID 1, but just as important for RAID 5 or 6. SO you need to understand your current array, and what the volume size of the disks is. If you go bigger drives then exists, then the space will be truncated to the size of the drives you have (though in theory, you can build another array out of the excess, it just can't be part of that array).
BUT, if you're planning an entirely new array, that isn't part of the existing array space, then yes, you can put any size drive you want into the expansion, but you will be defining a new array. Then you need to think about what kind of array you need, and why you are building it.
There's nothing magic so much about the Synology NAS (which are super sweet), but you need to more understand the RAID array you are working with, and what your objective in adding more drives to an array (or creating a new one) is.

Best Regards,
Scott
MSc ISM, MIET, MASHRAE, CDCP, CDCS, CDCE, CTDC, CTIA, ATS

"I try to be nice, but sometimes my mouth doesn't cooperate."