The approximate size of the (Rakatan) Star Forge

The first of the following links shows us the relationship between the maximum width of the Star Forge and that of its control tower.

The second compares the width of the control tower with an object of known proportions (the Hammerhead cruisers about to blast the heck out of it). Though there are inaccuracies due to the cruiser being closer to the camera than the tower, I wouldn't expect these inaccuracies to increase the total size of the Star Forge by more than 50%

 

First image tells us that the station is 72.5 times wider than its control tower.

Second image tells us that the tower's width is roughly half of a Hammerhead's length. A Hammerhead is apparently 315 meters long, according to Wookieepedia, so the tower is roughly 155 meters wide.

That leaves us with a total width for the Star Forge of slightly over 11 km and an height of around 28 kilometers, give or take a kilometer in either direction. Actual figures might be slightly higher, but for ballpark figures these are pretty good.

Sources :  http://forums.spacebattles.com/threads/the-approximate-size-of-the-star-forge.215559/
et http://starwars.wikia.com/wiki/Star_Forge