We report on 3.6 cm VLA and VLBA observations of YZ CMi and AD Leo , two nearby dMe stars .
We resolve YZ CMi and can fit a circular symmetrical gaussian component of FWHP of 0.98 \pm 0.2 mas , corresponding to an extent of the corona above the photosphere of 1.77 \times 10 ^ { 10 } \pm 8.8 \times 10 ^ { 9 } cm or 0.7 \pm 0.3 R _ { \star } ( R _ { \star } refers to the photospheric radius ) .
We obtain an estimate of the brightness temperature of 7.3 \times 10 ^ { 7 } K , which is consistent with that expected from gyrosynchrotron emission .
For AD Leo the emitting region is unresolved .
We therefore set a conservative upper limit to its diameter of 1.8 times the photosphere diameter , which leads to an extent of the corona above the photosphere of < 2.8 \times 10 ^ { 10 } cm or < 0.8 R _ { \star } .
We compare the radio emitting dMe stars with measured sizes with the Sun and conclude that these active stars have much more extended coronal radio emission than the Sun .
The VLBA position of YZ CMi has been found to differ by 32 mas from the positions calculated from the Hipparcos catalogue .
The discrepancy is caused by large errors in the listed proper motion .
An improved value is given .