The resistance of a length of metal is given by:

Where ρ is the bulk resistivity of the material, L is the length, and A is the cross-sectional area as shown in the figure above. Sheet resistance R_s is given by:

If a material has a sheet resistance of 10 ohms/square and is cut into a square shape (L=W), then the measured resistance from one edge to opposite edge will be 10Ω regardless of the size of the square.

See this and more important radio frequency charts and formulas in this comprehensive application note.