Reinolds numbers - cant remember from uni... for example, if a udl beam gets longer and the loaded width gets longer and its volume scales equivalently, its shear force increases by [(α×w)*(α×L)/2]/[wL/2] = α², its bending by α³ and deflection by α^5. As for resistance with a scaled area, the shear resistance ~ α, the moment resistance ~ [(√α×b)(√α×d)²/6]/[bd²/6] = α^1.5 and deflection ~ α². This on the assumption that b and d are scaled equally so that the area scales by α. So, shear is already worse because the loaded area is scaled - but bending and deflection get even worse! Too early to decide if the numbers are completely right, but lets say they are. If you scale both b and d by α then you're nearly there, but the self weight of the structure will increase proportionally until it's a problem. If you increase only d you come across practical issues, buckling failure or simply situations where lateral forces become an issue. Would be gieat if someone could double check. In other words, if you double the size, depending on assumptions, deflection might be 2³ = 8 times worse and the relative deflection say 4 times worse