Like everything else in history, "it depends".

It is pretty easy to decipher where the term "rear admiral" came from: during the age of sail, he was the admiral in charge of the squadron of ships in the rear of a formation (in contrast, the vice-admiral commanded the vanguard). This rank evolved as time went on to being one of an admiral typically in command of a "division", meaning, a group of ships subordinate to a "squadron". A couple of divisions made up a squadron, and each fleet had a couple of squadrons. Generally speaking.

That being established, we have to understand the difference between rank and position. Rank is the bars, stripes, or stars on your collar or sleeve, position is the actual duty position that you occupy. In naval terms, this means that you have positions like captain (meaning, the commander of a ship) that are not necessarily billeted to be filled by captain-by-rank (O-6 and above, in modern terms). The same goes for rear admirals...they were flag officers, which meant that there were many important positions ashore and afloat that they filled. Over time, however, the idea of the "rear" and "van" in a naval formation gradually were replaced by "divisions" and other such nomenclature, so too did the position of a rear admiral change. You're asking more about the position of rear admiral, versus the rank.

To answer your question, then: in the largest formations (The Grand Fleet, the High Seas Fleet, Kido Butai, TF16, etc), that featured numerous capital ships, a rear admiral would command a division, which nearly always comprised of two or three capital ships. Typically, RADMs would also command groups of smaller ships: light cruiser or destroyer divisions, small task forces (such as those based around a heavy cruiser or a light aircraft carrier) too large to be commanded by a captain or commodore. Rear admirals also commanded submarine flotillas and fairly large regions of coastal or territorial waters.

In the Royal Navy, a Rear Admiral would typically command a cruiser squadron, or a division of larger ships. However, Rear Admirals would sometimes find themselves commanding squadrons of capital ships. They were also employed as second-in-command for squadrons commanded by those of higher rank. There were also positions such as 'Flag Officer, Destroyers', which represented administrative, if not tactical, control of all vessels of that type attached to one of the RN's fleet commands, such as Mediterranean Fleet or Home Fleet. They were usually held by a Rear Admiral commanding one of that command's cruiser squadrons. The RN's submarine force came under the command of 'Flag Officer, Submarines', a position frequently held by Rear Admirals. Rear Admirals could also hold command of the RN's overseas stations.

During the pursuit of the German battlecruiser Goeben in 1914, the 1st Cruiser Squadron of four ships was commanded by Rear Admiral Ernest Troubridge, while at the battle of Coronel, Rear Admiral Christopher Cradock commanded the 4th Cruiser Squadron. At Jutland, the RN's battleship divisions were either commanded by Rear Admirals, or by the Vice Admiral commanding the squadron of which they were a part. The sole exception was the 5th Battle Squadron, attached to the Battlecruiser Force, which was commanded by Rear Admiral Hugh Evans-Thomas (though this force contained as many ships as the typical division). Each division had four battleships, with the exception of the 3rd Division which contained the fleet flagship, which operated with the division but wasn't a part of it. The battlecruiser squadrons and armoured cruiser squadrons were all commanded by Rear Admirals. The two armoured cruiser squadrons had four ships each, while there were two three ship battlecruiser squadrons and one two ship.

The RN entered WW2 with Rear Admirals commanding Home Fleet's 2nd Battle Squadron, Battlecruiser Squadron and 18th Cruiser Squadron. Rear Admiral Holland, who would be sunk with the Hood, commanded the Channel Force, with 7 capital ships and a destroyer flotilla. Rear Admirals would command Mediterranean Fleet's battleships and carriers, as well as its cruisers. They would reprise the latter roles for the Eastern Fleet. Rear Admiral Cecil Harcourt commanded the 11th Aircraft Carrier Squadron in the British Pacific Fleet. Rear Admirals in WW2 could find themselves directly controlling anything from a single carrier or two battlecruisers to eleven cruisers.