The size of Spacedock is a matter of some conjecture. Mike Dicenso was kind enough to post me some production sketches which show the station as having a diameter of 12,500 feet and a height - excluding the towers/ariels on the top and bottom - of 15,250 feet. I have converted these to metres for the page; the overall height is calculated from the scale diagram used in the size comparison link.

Much debate concerning Spacedock concerns the size of Starbase 74. The same model was used for both stations, but the Enterprise-D easily fitted through the space doors of the latter station; yet Spacedocks doors only just took the original Enterprise, a much smaller ship. Many consider this latter point to be a nit in itself, since in ST III the significantly larger USS Excelsior followed the Enterprise out of Spacedock.

To calculate the exact size of the doors, I took three scans from Star Trek III. The first was of the Enterprise backing out of the space doors; this is by far the best shot in the film in terms of a close up, square on view. Perspective distorts the sizes slightly as the doors begin significantly further from the viewer than the ship does,
and end significantly closer. I measured in line with the saucer, and got the following sizes :

Saucer diameter (pixels) = 100
Door width (pixels) = 183
Ratio (door : saucer) = 1.83

Actual size of saucer = 140 metres.
So actual door width = 1.83 x 140 = 256.2 metres.

Just a few seconds later, we get a view of the Enterprise turning to move out into space as the Excelsior begins its pursuit. Since we are a good kilometre away at this point, the perspective changes are minimized - but the measurements have a greater degree of inaccuracy, since we are measuring smaller distances. There is a considerable degree of perspective distorting the shape of the doors, and I measured the width at the same height as the Enterprise saucer.

I capped two shots of this scene, to see if I got the same results at different points during the turn. These are the numbers I got :

Capture 1

Saucer diameter (pixels) = 44
Door width (pixels) = 91
Ratio (door : saucer) = 2.07

Actual size of saucer = 140 metres.
So actual door width = 2.07 x 140 = 289.5 metres.

Capture 2

Saucer diameter (pixels) = 43
Door width (pixels) = 93
Ratio (door : saucer) = 2.16

Actual size of saucer = 140 metres.
So actual door width = 2.16 x 140 = 302.8 metres.

Now I would tend to trust the first figure more than I would the other two, because we don't know just how far beyond the doors the ship backed before it began to turn - even a couple of hundred metres could affect the results significantly. The ever increasing sizes would support the ship still moving backwards as it turned,  hence gradually appearing smaller.

So I would take a door width of 256 metres to be about as close as possible.

Now, regarding the Excelsior : according to Encyclopedia, this class is 467 metres long. But the DS9 TM gives a length of 511.25 metres for the Excelsior - both lengths are given next to side on images of the Enterprise-B subtype.

The Encyclopedia also gives a top image of the same type, and from this I worked out a beam of 184.8 metres from a 467 metre length, which rises to 202.3 metres with a 511.25 metre length. However, the DS9 TM specifically states that the beam is 195.64 metres.

I tend to dismiss the DS9 TM entry, as this section of the book is riddled with mistakes. However, the beam of the Excelsior is certainly somewhere in the 180 - 210 metre range, and a a 256 metre Spacedock door would be more than capable of handling the upper limit of this range, while fitting in with everything we see in ST III.

For details concerning Starbase 74, see the comments page of that stations entry.

Spacedock could be a totally unique design, but I think that's pretty unlikely. Given her size, I doubt more than fifteen or so Spacedocks have been built in all - especially with the much larger Starbase 74 type out there.