I just shucked my Seagate Expansion 4TB portable HDD hard drive and I was looking for another enclosure to house the drive. However, all enclosures I've ordered only fit the 2.5" SSD/HDD size, and other enclosures are made to fit the large internal 3.5" HDD drives. When I measure the shucked Seagate HDD, it measures to about 2.75" x 4". The issue is that the drive is 0.5" in height vs. an SSD at 0.25".
As weird as it sounds, 2.75 inches is 2.5 inches, so your hard disk drive will go into any 2.5 in. enclosure.
There is the Wikipedia page which lists disk form factors:
2.75 in × 0.197–0.75 in × 3.945 in [...]. The 2.5-inch drive format is [...] widely used for HDDs in mobile devices (laptops, music players, etc.) and for solid-state drives (SSDs), by 2008 replacing some 3.5 inch enterprise-class drives.
In order to be sure, I even grabbed an old SSD, SanDisk SSD Extreme PRO 240 GB 2.5". It measures indeed 2.75 inches.
See also a question I posted on SuperUser: Why 2.5 inch disks are 2.75 inch wide?