Since it came up in the comments, I'm describing how to make a baneling to use as a paper weight (if you feel so inclined). Using this as an example, I'm going through my step by step preparation and creation process that I only briefly outlined the other day.
I think this should be an easy entry into sculpting some custom SC2 figures, something as complicated as the ultra is maybe just a bit too frightening as a first project.
Step 1: Getting reference pics
I started up the map editor and placed a couple of banelings. I made them face in different directions and shot close up pics with PrintScreen. These are saved to your User/My Documents/StarCraft II/Screenshots folder and helpfully called "terrain xxx.jpg". If you position the camera close to the ground at just the right height, everything but the unit will appear black, which makes it easier to remove the background from your model pics with a magic wand graphics tool. The top and 3d view are a bit more hassle, but doable. I strongly recommend saving the pics in a format that supports transparent backgrounds, such as PNG (instead of jpg, which colors the background white).
This picture shows the contents of my "Baneling" project folder - terrain shots and the cropped single views with background removed:
Step 2: Figuring out the actual proportions and size requirements
Since each of the screenshots is taken from a slightly varying distance, the sizes do not quite match. This is the first thing you have to fix. I usually load all the PNGs in Inkscape and put them all on a base line. This is usually easy, because most objects have a clear lower limit (such as feet or in this case claws):
Next I figure out the topmost point of the object, which in this case is the highest spike on the back. Now draw a second straight line and proportionally adjust the size of all pics to just barely touch this second line (in most cases: hold ctrl while scaling):
Of course there are small perspective distortions, e.g. compare the width of the front vs. the back view - they should have the same width in an isometric view. Fortunately they are either negligible or can be amended with some common sense while sculpting.
Once you have your baneling views lined up and all at the same size, you can measure the proportions of width to height to depth. This will tell you how big of a stone block you will need. I usually draw a rectangle around the side, front or back and top views. Since the scale is close to identical, the length in the top view should match the length in the side view. This gives you a fix on the proportions:
For the baneling and the screenshot size I used, the proportions are 686 pixels of length to 380 px height to 462 width. Since you probably don't want to make a baneling exactly that size in whatever unit and since those proportions are too complicated, I simplify them with a bit of rounding.
length | 686 px | 343 | 114 | 16.3 | 8.2 cm <-
height | 380 px | 190 | 63 | 9 | 4.5 cm <- chosen scale
width | 462 px | 231 | 77 | 11 | 5.5 cm <-
: 2 : 3 : 7 : 2
The second to last ratios sound good and simple, but I decided I want the baneling to be substantially smaller than the ultralisk and a nice size for a paperweight, so about 8 cm in length should be good (roughly: width of a hand). In summary: we'll need a stone block of about 8.5 x 5 x 6 cm size. The raw stone should be in the ballpark of 750 g weight, the finished sculpture between a third and maybe half of that. Sounds about alright for a paperweight.
As a rule of thumb, you'll probably struggle with a accuracy of better than one millimeter. Then again, if you make the baneling too short or too long by 2 mm, that's less than 3% of error - hardly noticeable as long as it's not a human face. We're too damn good at knowing what faces look like, which is why they often turn out looking crappy in sculptures if you make even just small mistakes. Fantasy sculptures are much more forgiving. For example, no one complained that the blades of my ultra are at least a third too short, it's just not that apparent.
Step 3: Preparing the cheat sheet
I always prepare a cheat sheet summarizing what I want to sculpt. I find that during the initial sculpting phase a side, top and front/back view are totally sufficient. The baneling is simple enough that this might generally be enough. One thing I find helpful is to have a grid overlaid on the pics. It's even better, if the grid matches your ruler, ie. it's somehow compatible to inches or centimeters and not arbitrary units. You can achieve that by having a prepared grid that you can copy and paste in the future and then adjusting its size so that one of the measures of the baneling is just right (I used the height of 4.5 cm). The other measures should fit exactly. In my case, one of the images had a somewhat smaller length, since I had apparently taken that screenshot when the baneling animation had its ass less than maximally jutting out (as I intended). Easily fixed by a non-proportional lengthening (error was pretty small).
Link to full size PNG (A4, transparent background)
Preparation done! All in all, this whole process took about 45 minutes.
My plan is to carve this thing tomorrow (small, mainly blob shaped, not terribly much detail) and to later give it to a friend for his birthday. Polishing might take an additional session or two. I'll try and give you a full step-by-step report.
I'm grateful for all feedback, especially your own results!