Whats a bubble?
Looking from the side:-
1 layer with a gap is a cylinder.
2 layers could be cylinder or truncated cone.
3 layers starts to look like a truncated cone and an inverted truncated cone.
4 layers exhibits some curvature.
Don't know what nozzle you are using but if we said the min layer height was 0.12mm then vertically you could get 4 x 0.12 = 0.48mm. But only if you positioned it for exact layer slicing.
However if you think how hard it is to print small items. These are small diameter perimeters and could easily be bridged especially if you have stringy filament. Also circles tend to come out undersize.
So my guess is about 1.5mm diam as a practical minimum. But they would probably print at about 1.2mm diam
