It depends on how whether or not you're still using Windows Home Server V1 (which is based on Windows Server 2003 Small Business) and have not installed a modified driver for the ICH9R SATA chipset.
Short answer - 2TB
Long answer...
With the behavior in WHSv1, out of the box, you're best off with drives smaller than 2TB, and which don't use the 4K sector "Advanced Format"
Windows Server 2003 creates partitions that are aligned to maximize usable space by default, which end up be misaligned with the 4K sectors.
Additionally, it creates MBR-based partitions by default, which can only be just shy of 2TB in size.
It supports GPT-based partitions and drives, but they have to be configured with extra parameters. (Generally manually)
When the Drive Pool aspect of WHS adds a drive, it uses the default behavior for everything, so, you only see 2TB of your larger drive, created in a potentially misaligned MBR partition.
It's possible to use larger drives, but if you want them to be available as part of the MediaSmart/WHS storage pool, then you have to jump through a lot of hoops involving stopping services at just the right time, modifying the registry, and, if using 4K-sector advanced format disks, a modified* SATA driver.
*Intel, for some reason dropped the ICH9R from the list of devices in their version of Rapid Storage after 9.6, which keeping the ICH10R, and ICH7R that had been included side-by-side in 9.6 and prior, and for some reason, also keeping the ICH9M.
It's possible to add the line of text that references the ICH9R from the 9.6 driver, to the same text/.inf file in the 11.2 driver, but then the driver package is no longer "signed", so it will complain about installing
Newer versions of Windows (Home) Server, and Windows in general, as well as various Linux or UNIX options should be able to handle 4K-Advanced Format and GPT-partitioned, larger than 2TB drives without much issue.
All that said, the EX495 is much more power-hungry, and harder to repair than a more modern Intel or AMD APU-based system can be, especially since you have to buy or build a custom adapter to connect to a header on the motherboard to get a display, keyboard and mouse connected directly to it for troubleshooting.
The single slot for RAM only supports 4GB, and the power supply is wired in a non-standard way, making it hard to replace if your larger drives happen to draw more watts than the aging unit can support.