Virtually ANY waveguide can be reduced to less that three inches long

Directional Couplers in Waveguide sizes from From WR90 to WR 2100 

Connecticut Microwave "Loop" type waveguide Directional Couplers such as this WR187 BiDirectional coupler while not as broadband as broadwall couplers offer an economical price and small size. 

 In house machining and fabricating capability mean short leadtimes