More specs are required.
Do you want a 1-phase or a 3-phase? If its 500va then I guess you'd want to gor for 1-phase inverter.
Whats you're DC bus voltage?
What do you need the inverter for? Is it for UPS applications or motor drives applications?
you can use type 16 core 3 inch stack with windings primary 16 + 16 turns for 12v 0 12v using 12 swg
and sec use 330 turns (240v) using swg 20 ( 21 is also ok)
Added after 9 minutes:
thecalculation i have given is for 500w
for 500va around 300w
39 turns center tap for 12 0 12v
using 13 swg
sec 405 turns 240v swg 22 ( 23 will also do )
core type 16 2.5" stack