Yes, they can absolutely create smaller OLED monitors. The problem is that they will not with the current size of the silicon wafers used in the current generation manufacturing plants. The TV's and monitors are all laid out on this wafer and then cut up into the smaller panel sizes that then get used in the TVs and monitor manufacturing. The various sized panels currently being generated have all been optimized to fit on the wafer with minimal loss of surface area of the wafer. While they could certainly cut 24" panels instead of the 27" ones, they would then be losing all the surface area that could have been used to make the 27" panel, but there is not enough space in the layout to otherwise make use of it (i.e. there isn't enough to make an additional panel as the unused portions can not be oriented in a fashion to fit an additional panel with the remains).
Now it a case might be made at some point to figure out a new layout and orientation if the mix of panels they want to produce changes (i.e. they decide they want to make a different mix of sizes and they find they can fit the 24" panels more optimally...). However, I don't see that happening on the current generation plants. It might be introduced when they have the plants needed for producing larger than 77" QD-OLED panels (which should occur with the next gen plants), but again it is not a given that it will happen, only that it could and most likely would occur at the introduction of a new wafer size when they need to create the layouts of all the panel sizes they will manufacture with it.