Yes I am from "Moesia Superior" how sizable do you think it was?
Difficult to say. The Herulian kingdom stretched somewhere between Ostrava in the north, Vienna in the west, the Danube to the south and the Tisza to the east. That's quite an area. Not all Herulians resettled from there, and not all (probably not even the majority) of those who resettled were genetically Herulian, i.e. of the old Scandinavia / East Germanic stock - they surely picked up a lot of other people on their way. However, I guess the old Herulian elite was more likely to resettle, while the traditional inhabitants of north Danubian lands didn't mind that much whether their rulers were Goths, Herulians, Lombards, Gepids or whichever other Germanic tribe happened to establish a kingdom there. In any case, that was old Marcomannic lands, some "locals" that joined the Herulians, e.g. for family reasons, may also have been Germanic.
Sigidinum seems to have been quite a large city (you actually should be able better than I to find out how large, I suppose some Yugoslavian / Serbian archaeologists have researched on it). Moreover, the East Roman document speaks about cities - i.e. more than just one or two. In the 510 peace treaty with the Ostrogoths, East Rome had gained the easternmost part of Pannonia, the Bassiana civitas (near to today's Putinci, see map below), while Sirmium (Sremska Mitrovica) remained with the Ostrogoths. It is commonly assumed that the Heruli colony included Bassiana, so it formed a buffer against the Ostrogoths to the west, and the Gepids to the north. It surely also included Tauruno (Zemun). As such, the western extent of their settlement would have gone to a line that runs from somewhere west of Cusum (Novi Sad - Petrovaradin) southward to the Sava.
The eastern extent of the settlement is more difficult to estimate. The next larger Roman city on the Danube was Viminacium (Stari Kostolac). Being the capital of Moesia Superior, a transfer to the Herulians would surely have been explicitly listed, which it wasn't. As such, their realm must have ended somewhere west of Viminacium. The Tabula Peutingerania (picture below) displays two towns between Singidinum and Viminacium: Tricornio (Ritopek) and Monte Aureo (Seone near Smederevo). The Great Morava is just indicated as river 13 miles east of Monte Aureo (Margum fluvium), without a town being located there. Geographically, it would have made sense to select the Great Morava / Jezava as eastern border of the Herulian territory. If the new federates proved a bit less reliable then expected, East Rome would still have a well-defendable border of apparently uninhabited swampland at the mouth of the Morava, and a strong garrison placed in Viminacium. However, East Rome might also have chosen to maintain control over the Great Morava, probably a relevant transport route, and end Herulian lands somewhere between Tricornio and Monte Aureo, e.g. at the Danube bend near Grocka.
There weren't any major Roman settlements in the hills south of Belgrade. Furthermore, the East Roman historian Priskus reports that Attila had demanded Rome to depopulate all lands south of the Danube between Sirmium and Novae (near Svistov in Bulgaria) over a width of five day marches (~100 km) as a precondition to peace. Whether that just referred to military settlements, or to all the population, is unclear. However, with the soldiers' protection and purchasing power gone, many civilians should have left as well. As such, one can assume that at least the northern part of this 100 km wide depopulated stretch was given to the Herulians. I think the minimum southward extent was to the Ralja river, and then some line from the Ralja's source to the Sava around Baric. The theoretical maximum would be to the Western Morava, from there to the Drina, and then up north along the border with the Ostrogoths, corresponding to the western part of Diokletian's Moesia Prima. [
I suppose the East Roman acquisition of Bassanae was part of a general border adjustment, so the new border may have run more or less straight north-southward from Sabac to Roganica.] A southern & western border along the rivers Belica, Jasenica, Ljig and Kolubara would correspond well to the 100 km wide depopulated zone south of the Danube. It also reflects the assumed extent of the Tricornienses subdivision within Moesia Superior that was created by Emperor Trajan with initially Tricornio, later Singidunum as administrative capital.
How relevant such borders were in practice is another question - you will need to check with local archaeologists whether there is any indication of new settlement during 510-537 AD, or the area remained as sparsely settled as it was before. It could also make sense to check out Mt. Rudnik - if mining was still on-going there in the early 6th century, East Rome might have preferred direct control.
By 537 BC, the Gepids took over many areas along the Danube, and relocated their capital to Sirmium. This may have been part of a deal between with East Rome, which at that time was fighting the Ostrogoths in Dalmatia and Italy and needed a stable northern border. Since many of those Herulians that didn't relocate to Moesia Superior had searched refuge with the Gepids, I could imagine that transfer to Gepid rule (possibly more nominal than de facto) met little resistance. As such, the Wikipedia map above (as reliable as such maps are!) may also help to assess the southward extent of the previous Herulian colony. Gepid rule lasted until 567. Then, the lands south of the Danube briefly returned to East Rome, possibly as part of a deal with the Avars who had received East Roman financial support for their war against the Gepids. In 582, the alliance with the Avars broke. Sirmium and Singidunum changed hands several times before ultimately falling under Avar rule around 625 AD. East Roman historians attest the presence of a sizeable number of Gepids among the Avars by 630 AD, where exactly is unclear though.