Yes, larger airplanes can be built; however at that size, some adjustments need to be made.
For instance, the light of an aircraft is a function of the wing area, which increases as a function of the square of the change in size (for instance, and airplane 25 m long would have 1/4 the wing area of an exact copy of that plane built at twice the size, i.e. 50 m long). But normally, the mass of an airplane will grow as a function of the cube of the length; so a much bigger airplane needs a proportionally larger wing.
Very large airplane may need to explore different configuration than th traditional cylindrical fuselage with wings in the middle and tail in the rear; like span loader or flying wing.
Also, one has to consider the economical appeal of much larger planes. A flying wing would have essentially no windows; would passenger accept this? A different configuration airplane may require changes in airport infrastructure, already some modification were required to accommodate the A380, and those could be quite costly. Would this reduce the market appeal despite more efficient fuel usage?
So, yes, it can be done; but perhaps the world is not ready for it yet.