I think unreal for single person is limitless, ie. you never produce enough to exceed longint size limitations.
I once estimated sizes and scales for earth (to see maximum scale of planets in accurately represented solar system), where i assumed 1au equal to half of size of grid area.
Longint makes possible to put 20000 x 20000 x 20000 cubes that are size of grid.
So solar system made to the scale of 2au = grid size was still quite small compared to whole longint space.
There is small problem however, that not everything in unreal works outside grid, i think physics and replication start do weird things. Also animation start to have rounding errors.
But you have lifetime you can learn C++ and improve (or fix those places, get them merged into main branch etc). So assuming you do not want to exceed 32bit limitation, you still can do huge world. Also because games like elite dangerous (i think) implemented 64bit, there may be reason for Epic to consider this (maybe not necessary, but good marketing point). So there is big chance that at some point coordinate system of unreal goes 64bits everywhere. That probably would make possible to create world that spans light years in size (when you keep my test scale of 2au=grid size).
Another estimation to visualize scale:
earth surface is 500 mil square km. Assuming you make square of it, it will be ~22400x22400 km (same surface as earth surface on ze ball).
now integer is from −2,147,483,648 to 2,147,483,647.
If you take 1uu = 1cm (like Ue4 now assumes) you get 1.9173 of earth squares side by side in longint of coordiantes.
That makes whole grid 3.83 times of earth surface when you do 1uu = 1cm.
But it is not all, you probably could stack layer of such surface one above another and turn them on/off with script, or LOD).
So there is no chance you can ever reach limit of filling up single unreal map to the max. Realistically your limit here is your PC, sdd size, ram size etc.
But then you can do levelstreaming which is only limited by size and speed of hdd.
I am quite sure that doing it alone or even with 100 of other people, you will fill up that world slower than hardware power grows and slower than epic improves engine.
If i was you I would heavily dig into proceduraly generated maps, its bit harder than making it all manually, but then you can truly do infinite worlds.