A rack unit or one U is the standard size for servers and other network equipment placed in 19-inch server racks.
These racks accommodate equipment 19" wide ( or 482.6 mm ), and have a variable height, which is expressed in rack units. 1U is 1.75", or 44.45 mm.

If you want to place the hub in a 19-inch server cabinet, select 'yes' here.