Unless you want to use a sound card or external DAC and a separate amp, you will not be able to drive speakers as large as you want from your computer.
I just bought a Corsair SP2500 2.1 computer speaker system. The satellites are only roughly 4"x6"x6" but give surprisingly good sound for their size. The midrange is 3" and the tweeter is 1" and are bi-amped (each driver has it's own amplifier). The sub is only 8" but it is in a monster enclosure (roughly 10"x11"x18") that mounts the driver in a sealed camber to the back of it and a ported duct in the front chamber. I can feel the air moving in the port when I crank up the sub volume a bit. Just running on onboard sound, the Corsair sounds much better than my old Logitech X-540 5.1 system, which sounded much better than anything Bose had for the same price. I recently picked up an Asus Xonar Essence STX sound card to see if it gives better even better sound than the onboard Realtek sound on my Asus P9X79 WS MOBO which the reviews claim it will. I'll be temporarily installing it in a day or two to evaluate it.