Baffle should be less than 2 * effective cone diameter, so in the case of that small FR it could be quite sleek. The rolloff is then compensated actively using SLP. A minimum of one dipole peak notch is required.
Musicanddesign's ABC dipole simulator ($15) is really great and I've used it with success. For that Peerleess it goes like this.
10x30cm baffle, driver at 20cm
Typical one dipole peak which needs to be equalised after which the response is smooth. The simulator cannot predict the dipole peak accurately, it must be measured outside.

25x40cm baffle, driver at 20cm height
After the dipole peak the frequency response is crap, that large dip and peak is difficult to equalise.
