I suppose everyone's got a preference, I typically try to spread my research out to cover multiple bases, but tend to take larger hull designs later. Maybe a little slower getting there, but once it's done, it's hard to beat. Miniaturization doesn't do you any good if you haven't researched more efficient weapon and engine designs, IMO.
A fleet of 4 Small fighters packing 15 damage (Last Phaser x3), speed 11 (Last Warp Engine x2), and 1.7 range works pretty dern good against any single ship, no matter the size. 'Corse, that'll change as I get newer weapons in the Campaign mode.
Once you've got (somewhat) disposable small fighters flitting around your systems holding the AI at bay, you can research and build larger ships to really put the hurt on, and divvy up those experienced Small ships with heavier mediums or large ships to act as brute force bashers.
I guess no matter what you want to do, cranking the research to 70% and letting the social/military sit at 15% each helps get you there faster. I'll always take a hit in productivity to start to gain a technological edge early and keep it.