I would set the fronts to large – the 4T can reproduce sound down to around 45 Hz and setting the fronts to small will set the crossover cutoff at 80Hz. Setting them to large will send the full range to the 4T.
Set the crossovers on the IWs to 80 Hz. Same with the 4C center.
As for the levels, your best bet is to do a manual calibration with an SPL meter. Since you have a 5.1 system from us, you should have received one with your care kit. Use that to set the levels to balance the system. We definitely recommend a manual calibration over an automatic.
Enjoy your new system!
The 4T do go surprizingly low and I experimented quite a bit with them but in my 2.1 (4Ts and 8D) case the system sounds best with the 4Ts set small and crossed over at 80Hz. Another benefit is that the sub is always on all the time the AVR is and only shuts off 15 or so minutes after the AVR is shut down. Of course the spl meter can be quite helpful, I have just never gotten around to getting one.