Take 1E-13 solar masse as target regime. Einstein radius is 3E=7 smaller than solar mass MACHO.
Quasar ulensing paper from Kochanik has this table:
Lens R_e is for 0.3 solar masses. We are down by sqrt(1E-13/0.2) = 5 e-7. So for a favorable source, lowest mass lensed QSO, we have R_e=7.8e9 cm, and Rs=2e12. Ouch.
We need a more compact source. So use lensed SNe! Typical timescale is dominated by lens motion not source motion, and is 10 years * 5e-7 = 5e-6 yers x 3e7 sec/year = 150 sec.
Now we need an estimate of event rate. Note that apparent angular size is increased by lensed magnification so we don't want highly magnified lensed SNe.
Diego et al (20220 in paper about lensed type Ia say that photosphere radius after 1 mo is 10^-3 parsecs, or 3E15 cm. Too big! In retrospect this had to be true.
If these objects have comparable spectra, then to have comparable magnitudes their emitting areas have to be comparable.