The exact center coordinates and the angular sizes of the nebulae are probably largely irrelevant. I'm not sure how large your antenna is, but the angular resolution of individual radio dishes is typically pretty poor (hence the practice of radio interferometry in radio astronomy). For instance, if you are observing at a frequency of 1 GHz with a 3 meter dish, your angular resolution is about 6 degrees on the sky. In general, your resolution is inversely proportional to your dish size, with
$\theta \sim \lambda / D$,
where $\theta$ is your resolution in radians, $\lambda$ is the wavelength, and $D$ is the diameter of your dish. If your source is smaller than your resolution, then the large scale structure does not matter.
If you take any of the best northern hemisphere options, Cas A, Cyg A, or Taurus A, and point to the coordinates you find on wikipedia, you should do just fine for calibrating your antenna.