Now a days when I work with 65mlnPx images from Hasselblad and PhaseOne - yours brush looks tiny. As a poweruser and betatester I now how to change it manually inside the application code - but I wish that this feature should be for everyone.
We have the upper limit set arbitrarily to a value that is still performant. As we see improvements to hardware/graphics cards, we'll look at allowing bigger brush sizes while maintaining performance.
1500 pixels is already slow enough on all but the fastest workstations (and those too).
I think working on a smaller copy and saving the mesh to apply to the full-res master is still the best workaround, especially for the majority of users who ain't gonna mess with no application code. Probably faster than waiting for a 5000px brush to make its way across the mesh.