I have never heard the term before, so will be keen to hear other replies, however since there aren't any yet, I'll speculate that it has to do with shear effects being of greater importance than they typically are.
Cheers,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
I wasn't familiar with the term either. But if you google "size effect in deep beams", there are quite a few papers which use the term. In essence, the deeper the beam, the earlier shear cracks form at a given shear stress. All this research is to better develop strut-tie models for design of deep beams.
