EFI and freebsd-boot

When creating and an EFI partition, I'm not clear when a freebsd-boot partition is also required.

Can anyone explain?
The freebsd-boot partition is for a traditional BIOS boot (called CSM with UEFI). It's not needed to EFI-boot a system (that requires an efi partition).
So if I create a USB stick and install FreeBSD on it, how do I ensure it boots on both systems? Or do I need two seperate sticks?
Does it matter about the order... and if I want to install GRUB will it work on uefi and legacy?
What sizes do I need for the two partitions?

AIUI the efi partition only needs a copy of /boot/boot1.efifat so shouldn't 800k be sufficient?
That is what the installer uses.

root@E6430:~ # gpart show da0
=>      3  2015221  da0  GPT  (984M)
        3     1600    1  efi  (800K)
     1603      125    2  freebsd-boot  (63K)
     1728  1429968    3  freebsd-ufs  (698M)
  1431696     2048    4  freebsd-swap  (1.0M)
  1433744   581480       - free -  (284M)