Migration from physical to virtual

Dear all Actually, we have a pair of BIG-IP 4000s with LTM-ASM-GTM-AAM license, and we are considering the migration to a Virtual Appliance instead of the physical appliances We are facing a sizing problem , what sizing is the best suitable to replace the 4000s and satisfy the computing needed by the ssl offload in terme of vMemory, vCPU , vDisk ..... Is there and sizing tool, doc and best practices Thanks a lot