Having used Blender for the most part, and learning Maya (heaven help me! ), and UDK in college, I can tell you straight away that UDK is a game engine, and it's toolset is nowhere near what you would want to use for modeling (Assuming of course that by UDK, you mean the Unreal Development Kit?).

What I would do, is on the object/character reference sheet, either place a single square near the corner with unit and size per square, OR put a semi-transparent grid over the image, and include the square units somewhere; then calibrate the 3D software's grid units and size to accommodate. You want the main grid lines to match up to your grid/square, and then add subdivisions.
You'll do this by matching the 3D software grid to either the single square or the semi-transparent grid.

(hope all that makes sense )
and good luck