What is the amplitude of the largest internal wave ever observed?
I believe the largest internal waves ever recorded in the ocean were observed in the South China Sea (SCS). The diagram above is from Alford et. al (2010) - see the two ridges near A1 and N2? The gap between them is just the perfect size so that the barotropic tide can interact resonantly and give rise to very long internal tides which radiate away towards MP2 and eventually shoal (they also go the other direction but we ignore that since it's more or less the open ocean). 

The internal tide is very long, but its amplitude is not that large. However, as the internal tide shoals it fissions into a train of solitary waves. The leading solitons in the wave train are larger in amplitude but smaller wavelength than the original internal tide that gave rise to them. Each soliton propagates at its own speed with the largest amplitude wave propagating faster. As the amplitude of each individual soliton grows, the fluid velocity will eventually exceed the wave speed and the waves will break due to convective instability (overturning). Thereby depositing their energy nearshore, far away from the generation zone.

The largest amplitude wave measured in this study was around $200$~m, measured at LR2. But I am pretty sure I have seen other studies with larger amplitude waves in the SCS.