[SteveG (Steve)]

Suzy asked if I would contribute some suggestions as I always include size or dimension estimates in my observing notes, so for what it's worth, here are my techniques.

The first method was just mentioned above. I keep a printed page in my notebooks (and sometimes attached near the eyepiece) that includes my various eyepiece focal lengths, magnification and actual field. Using the actual field as a ruler works reasonably well for large objects, such as large open clusters, that take up a significant fraction of the field. Just estimate the percent of the entire field (or the radius) and multiply by the field diameter. I find that this is easier, though, with small to moderate apparent field eyepiece designs such as Plossls or Orthoscopics. Now, I'm using Ethos eyepieces with a 100° apparent field that require peering around to see the edge, and this method is less accurate.

A second technique that's probably rarely used among amateurs is to let the object drift through the field, either by placing it just off the east edge or just on the west edge, and time (using a stopwatch) the number of seconds to enter or leave the field. With a scientific calculator (using a little trig) the seconds can be easily converted into arc minutes (hey, I'm a math teacher ;-)

A third method uses nearby field stars and works well with galaxies and nebulae between say 1' and 5' in size. Often, besides my written notes, I make a rough sketch (I'm bad at accurate sketching) of the object and include some nearby field stars. Then I'll add a comment such as "extends 2/3 the distance between the two 10th mag stars just south". Later when I'm home at my computer, I'll look up the separation of these stars using a program such as Megastar or online with a program such as Aladin. Even this method sometimes fails, though, with compact planetaries and galaxies that are smaller than 1'.

The 4th method is to calibrate your eyepieces to estimate small sizes under 1'. How so? Well, I often start an observing session by taking a look at a few well known double stars that I can find instantly and observe them with the same eyepieces I often use for small galaxies. With my 18" scope, I often use 225x and 285x, and I'll quickly observe 2 or 3 doubles that I know the separations to calibrate my eyes. For example, in the summer/fall northern skies of California, I'll look at Albireo (35" separation), Polaris (18" separation) and Gamma Delphinus (10" separation). By doing this I have a pretty decent sense of these separations using those two eyepieces and feel comfortable making an estimate for a small planetary or galaxy as say 30" diameter. If the object is elongated roughly 2:1, then I log it as 30"x15". My experience is that at 285x, a galaxy that is just 30" or 40" diameter appears larger than most amateurs would probably guess, so it's easy to overestimate sizes.

Hope some of these ideas help!