On the front panel there is a button called "speaker". Push it and the display should say "front speaker". If you push the "set" button you can then use the left and right arrows to change from small to large. Push "set" again to save your changes. The panel goes from front, center, surround, surround back and subwoofer by pressing the left and right arrows. Not nearly as easy as using the OSD or remote though.
Did you lose your remote? Why would you want to do it that way?
I used to have a 320 but now a 325. I believe it works the same way on both.
One possible solution (that requires a remote, just not the original H/K remote), is to go pick up one of the "One For All" series of remotes, such as the One For All URC-8810. I just saw a bunch at Wal-Mart the other day for about $20. I use one to control my H/K 325 receiver along with 7 other components, and it's great (and cheap). Receiver set-up code '0110' should at least allow you control the on-screen display (using the 'menu' keys along with the four arrow keys and the 'select' button). I can also send you some of the advanced codes that would allow you directly select sources from the remote (e.g. CD, DVD, VCR1, etc.) if you decide to go this route.
BTW, I think that while the 325 allows you to set the speaker levels from the front panel, the 320 does not (they both have five small circular buttons on the panel, but on the 320 they are assigned to different surround processing modes rather than speaker and channel selection).