20A, with a good heat dissipation (ie, a relatively cool environment) doesn't require tracks that are too big. Plenty of tricks, like building up traces with sodler, or soldering on extra copper (either laser cut from thin sheet, or i've seen copper "braid" (like solder wick) used as well.
You can take a stab at the electrical resistance of your trace (plenty of online calculators) and estimate the heat flux. The important part is to keep the trace cool. I've done over 400Arms on a pcb trace before (ok, with a back cooled IMS pcb ;-)
Also important will be a low parastic inductance of the parts of the bridge that see high di/dt, to limit voltage spikes and the necessity for active snubbing or slow gate switching etc. Minimise the distance between the high and low FETS, which also helps limit trace resistance and power loss/upheat etc
As mentioned, using via stitching and multiple layers of copper can help, but for only 20A i don't think you'll have too much of a problem. It isn't that much more expensive to double the copper thickness on your pcb if you get into trouble with your design either...........