Selecting capacitor for voltage-divider of half-bridge high power SMPS
Capacitor choice is largely a matter of losses, yes. The capacitance value needs to be large enough to keep the midpoint more-or-less-fixed, with low ESR and ESL also being important. Under some transient conditions the midpoint can (and will) move around, so it's important to keep derating in mind as well. 50% voltage derating is well within reason (using 400V caps with a total DC input of 400V, for instance).

For AC-application capacitors (i.e. film caps), the ESR is often implied in the dissipation factor datasheet parameter, which is the relationship between ESR (resistive loss) and reactance:

\$ DF = \omega C \cdot ESR = \dfrac{1}{Q} \$

\$ \omega = 2 \pi f  \$

It will usually be specified at a given frequency, which allows you to calculate the ESR.

Electrolytic capacitors generally have large ESL by nature of their construction vs. film capacitors, limiting their usefulness at high frequency. Film capacitors also have some self-healing capability under dielectric breakdown scenarios which make them a 'robust' choice in half-bridge designs.