Over 100 oligoporphyrin (porphyrin molecules fused to each other through rigid acene-type bridges) molecules have now been synthesized, their long rigid pi-bonded structures making them very suitable as molecular wires while their synthetic flexibility offers the possibility of tailoring their structural and electronic properties to match specific needs. To examine their basic operational principles and to explore synthetic possibilities, we optimize the geometry of 85 oligoporphyrin and related molecules including porphyrin dimers and trimers using the accurate B3LYP density-functional technique. Also, a scheme is developed by which accurate geometries of oligoporphyrins of arbitrary size can be estimated, and this is applied to determine the geometries of a further 13 porphyrin trimers and tetramers. At these geometries we analyze SCF orbital properties in order to determine the superexchange electronic couplings within the oligoporphyrins. Couplings are monitored for bridge-length dependence and interpreted in terms of a detailed description involving bridge-porphyrin orbital resonances, as well as in terms of a simpler picture in which pi-electron delocalization is seen as a prerequisite for strong intramolecular coupling. Variations of the coupling with the nature of the bridge (e.g., naphthalene, anthracene, free-base or protonated 1,4,5,8-tetraazaanthracene, tetracene, pyrene, coronene, biphenylene, dicyclobuta[a,d]benzene, dicyclobuta[b,g]naphthalene, dicyclobuta[b,h]biphenylene, and bridges additionally fused to porphyrin meso positions) and porphyrin (e.g., porphyrin or bacteriochlorin, beta-substituents such as methoxy and cyano, Mg, Zn, Ru(CO)(2), and free-base porphyrins) units are considered, and the physical origin of quinonoid switching is determined. Terminal "alligator clips" such as fused phenanthroline, here complexed with (CuCl2)-Cl-I, are also considered.