Unidirectional motor control
You need to calculate the worst-case on voltage drop for your transistors and do a power calculation. For a BJT it will be the collector-emitter saturation voltage. For a MOSFET it will be determined by the on-resistance.

With this information the power is calculated by \$ P = V \cdot I \$ or \$ P = \frac {V^2}{R} \$. Compare this with the device ratings and calculate the temperature rise.

Given that weight is a premium constraint it sounds like a flying device. Given that you're using multiple motors I'd guess it's a quad-coptor. In that case you may be able to mount the switching transistors under the propellers and get some free forced cooling. A small TO-220 finned thermalloy heatsinks, for example, are very light and with forced cooling would dissipate a lot more heat than in a static environment.



Figure 1. Thermalloy TO-220 heatsink.