I personally like something like option 2b, albeit with a slower growth curve - each hull size is some percentage, say 125%, the size of the Last... I know about diminishing returns but it gets kind of silly when you get really high in levels you're spending ridiculous amounts of research points for minuscule improvements
... also, an exponential curve allows for a wider variety of ship sizes - MOO2 had ships ranging from 25 kT to 1200 kT and that was only 6 sizes
(OK, so they did double or triple every size...) The only problem with that would be making it feasible to actually design the larger ships... either you'd need scale mounts (which don't work when the largest ship is over 100 times the size of the smallest) or you'd be clicking a LOT of components on the larger ships!