Incidentally, there is no 2.5" hard drive installed ex works. There is only an empty slot with a tray for a 2.5" hard drive. A 16GB eMMC is soldered in at the factory. This is enough to record a good month of video material. When the disc is full, the overwriting process starts all over again.
The width is approx. 70 mm, the depth 100 mm and the height 15 mm. This means that standard notebook hard drives or SSDs fit in. It is important to consider how much and how often you want to write and read on it. With a Samsung EVO 870 with 4,000 Gb, you can probably record the entire year of a family home on one hard drive and still have enough space. For my part, I use old 2.5" notebook hard drives and overwrite them a few times with recorded videos.