  We have measured angular dimensions of 312 planetary nebulae from their
images obtained in Halpha (or Halpha + [NII]). We have applied three methods of
measurements: direct measurements at the 10% level of the peak surface
brightness, Gaussian deconvolution and second-moment deconvolution. The results
from the three methods are compared and analysed. We propose a simple
deconvolution of the 10% level measurements which significantly improves the
reliability of these measurements for compact and partially resolved nebulae.
Gaussian deconvolution gives consistent but somewhat underestimated diameters
compared to the 10% measurements. Second-moment deconvolution gives results in
poor agreement with those from the other two methods, especially for poorly
resolved nebulae. From the results of measurements and using the conclusions of
our analysis we derive the final nebular diameters which should be free from
systematic differences between small (partially resolved) and extended (well
resolved) objects in our sample.
