Smallest possible surviving (Western) Roman Empire?

[SlyDessertFox]

Its still around. We call it the Roman Catholic Church. The only place the Roman Empire retains temporal power anymore in in Vatican City.

The pope was never emperor. Nor was the papacy as we know it in existence in the 5th century or the 6th, 7th, or 8th.

 

[LSCatilina]

The pope was never emperor. Nor was the papacy as we know it in existence in the 5th century or the 6th, 7th, or 8th.

Nor that it did appeared overnight with Ottonian Reforms. It's more of a gradual construction, not unlike the building of western european nations.
Given the pope holds a title traditionally devoted to the princeps or dominus, pontifex maximus, that had a vital importance for roman identity...It's not that far fetched as you may think.

Yes, it was only one title, claimed back only after the fall of the WRE. I doesn't claim at all a continuity between Vatican and Roman Empire there (trough this or fallacies as the donatio), but at least a tentative of building a bridge (see what I did there?) between Roman tradition and pontifical power even in a period where it was particularly limited.

 

[DominusNovus]

It's doable, while the region is far less accessible for ERE, and that the abandon by the administration (instead of only emperors moving from different centers) would be still huge problems.
With, as you noticed, the issue that Aquilea was right in the way of raid and romano-barbarian campaigns. While it makes the city a perfect place for a QG (as Marcus Aurelius used it), it's far more perillous for a whole administration. (Without mentioning the problem of walls : they were destroyed by Huns in the 450's)

If only there were an easily defensible location nearby that could be built up into some truly impressive fortifications.