How should I size the transistors in the class AB linear amplifier
The parameters for FETs depend on your design specs for RdsOn (&lt;1% of load), Vds, load, Pmax, thermal margin, etc.
Cross-conduction failure MUST BE avoided. (both Nch &amp; Pch on at the same time)
With common-emitter, (CE) complementary drivers only one driver is active at a time. With voltage feedback, this eliminates the Vgs exponential factors and
eliminates the crossover-distortion with the Op Amp gain as error correction.
Each component in Class AB ought to be rated for power and temperature equal to the load so that at half power, you are derating your device dissipation and heat rise 50% which is prudent for any thermal design.
update
On second thought, it is possible to use PAIRED DRAINS with current feedback. But phase shift with high gain and stray reactive loads will need phase margin compensation.  (not shown)

Here is my design


I chose very low RdsOn (mOhm) power FETs with an ideal Op Amp to drive an inverting BJT to the FET with negative feedback to correct for all non-linearity.  I arbitrarily chose a voltage gain of 100 max.