For designing a runway, what are the largest cargo airplanes in use?
Size doesn't matter.*
If you are designing a runway, then you should get familiar with ACN and PCN.

The aircraft classification number (ACN) is a number expressing the relative effect of an aircraft on the runway pavement for a specified standard subgrade category.

* There can be the largest aircraft out there, but it may have a lot of tires, so the force it will exert [per tire] on the pavement might be lower than that of a smaller plane (e.g., A380 and MD-11).
From the linked Wikipedia article, the MD-11 stands out as the harshest on pavements (after discounting non-freighter aircraft).


Runway length, and wingspan
In most cases, aircraft are loaded according to the available length, not the other way around. Not all airports that handle 747s have 11,000-foot [corrected length] runways.
For the wingspan (or main gear span), the categories are defined by ICAO Aerodrome Reference Code.

Element 2 of the Code is derived from the most restrictive of either the aircraft wingspan or the aircraft outer main gear wheel span.