[eugovector]

So the Onkyo speakers are: 5 1/2" x 14" x 3 11/16"

I wouldn't actually consider those "big", but you want something smaller than those?

For real small speakers try the HSU Ventriloquist and Emptek ECA-4, those should both be within your budget for a 5.1 system with sub (SVS, HSU, Elemental Designs). Orb Audio is another possibility, but out of your budget.

 

[eugovector]

Another option would be the Intimus 4B Harmony SD set from Aperion. These are small, get great reviews and you can listen to them / look at them / convince the other half for 30 days with free shipping both ways if you decide they are not for you.
SVS, HSU, RBH, are some other options for excellence in compact speaker systems.

I actually have the Aperion 4 series if you'd like a firsthand review, however, those speakers, along with the SVS SBS and HSU HB-1 aren't much smaller than the speakers that come with the Onkyo 6100 (though, you get better sound than the 2 systems I previously offered).