The separation of challenging mixtures such as water/ethanol, CO/N, and aliphatic/aromatic amines requires tailor-made materials with ultra-micro porosity. Metal-organic frameworks (MOFs) are well-known for their unique properties, including pore tunability and useful functionalization. However, designing new ultra-microporous MOFs is difficult owing to the limited availability of short linkers. Here, we report the synthesis of an ultra-microporous zirconium-based MOF constructed from the bulky, pyrene-based 1,6-pyrenedicarboxylate (1,6-Pydca) linker. Contrary to the common strategy of shortening linker length to reduce pore size, the present work demonstrates that the lesser-explored approach of increasing linker steric bulk offers an equally powerful means of modulating pore aperture. Incorporation of the 1,6-Pydca linker yields an eight-connected Zr cluster with ∼4 Å pore window, thereby establishing linker width as a critical design parameter for ultra-microporosity. The resulting MOF exhibits highly selective sorption: preferential uptake of water over ethanol, selective adsorption of CO over N, and turn-off fluorescence sensing with small aliphatic amines (NH, CHNH, CHNH) in the presence of bulkier aromatic and aliphatic amines. These findings present an important dimension in MOF design, where linker length is no longer seen as a barrier to the design of ultra-microporous MOFs. This enables next-generation frameworks tailored for challenging gas/vapor separations and size-selective fluorescence sensing.