Allow my 2-cents in this. Ixx and Iyy are geometrically appropriate for beams, but for plates, the equivalent would be (width x thickness^3/12) for a slab. Plate theory expresses dimensions and loads in terms of per unit width (i.e., in-lb/in line moment), so the appropriate equivalent unit...
Depends on what behavior you are simulating...
is it in rope form?
is it a reinforcement in a binder?
is it layed up in a laminate?
is the laminate cross plied?
is the layup near "quasi-isotropic" with 0,90,+-45?
is CTE important?
is failure mode critical?
If the answers are yes to all...