Calculating H-Bridge without motor specifications
I recommend you use >6A DC-DC supply and 40A FETs 10mΩ or 1% of linear R min load to supply.  Supply ESR must be &lt; 1% of V/I so large low ESR e-caps are also needed. 1mF (10mΩ @ 15kHz) A battery of sufficient low ESR and Ah will have > 10k Farads capacitance.

My suggestion : RdsOn=1% of Vdc/Ir

Choose the following variables:   

\$\omega_I\$ = motor current commutation frequency
\$\omega_R \$ = typ. 82% max no load RPM
\$I_{R}\$  = Rated full load current at full power
\$V_{dc}\$ = Rated  DC voltage
\$Z_L({\omega_I})=\dfrac{V_{dc}}{I_R( \omega_I )}=\$
=  Motor  apparent full load impedance
\${I_{start}}\$  startup current at rated Vdc, (typ. \$I_{start}=10I_R\$+/-25%)
\$I_{max} \$ = desired max % of \$I_{start}\$  by controlling PWM or Vdc profile vs RPM  or field current
\$a_{pu}\$ [%] = \$\dfrac{I_{max}}{I_{start}}\$  desired or resulting max acceleration per unit of max in [%]
\$R_{on}\$ = MOSFET RdsOn in half-bridge
\$P_L{_{pu}}\$    = sum of all power losses as a %pu of total power at max rating
\$P_C{_{pu}}\$ = conduction loss due to \$R_{on}\$  of MOSFET in half-bridge with dead-time

Reasons

If RdsOn=1% of Vdc/Ir then Rdson = 10% Rm
If using a dual half-bridge motor [Rm + 2R_{on}] x # of phases
Thus 1% conduction loss at rated power and 10% of startup conduction losses (+/-25%) then Rm + 10% Rm  means startup current reduces 10% and bridges losses are equal to rated motor power but only for brief startup time. So heatsink must handle this and be current limited/ thermally protected.