Ok - I researched this idea a bit.
I am very used to ~250 MB to ~500 MB max for /efi, so the suggestion 2GB to 4GB took me by surprise.
You noted “grub-bls” or “systemd-boot”. … < sigh > me being the ‘old dog’ I definitely need to try and learn these new tricks, as opposed to being the ‘old dog who can’t learn new tricks’. 
My understanding now after some research follows.
Reference ‘systemd-boot’. I read it may be a more simple UEFI boot manager that also needs to store kernel/initrd files in the EFI partition.
Reference BLS: I read “BLS” is “BootLoader Spec”. Traditionally GRUB2 stores its config in /boot/grub2/grub.cfg, but BLS (currently used by Fedora ? ) stores kernel boot entries as individual files in the EFI partition (/boot/loader/entries).
If later SUSE GmbH or even myself becomes interested in exploring ideas (unlikely for me but one never knows) and decides to adopt BLS or systemd-boot, a small EFI partition (~500 MB) might quickly fill up quickly with:
Multiple kernel versions.
Initramfs copies.
Bootloader redundancy (for recovery
Hence both systemd-boot and BLS require more space in the EFI partition.
Possibly even systemd-boot and BLS are under consideration by the SUSEGmbH team for a future version of openSUSE LEAP ?? I don’t know. But I do like to be prepared.
I am thinking for an external, 2 TB nvme SSD, that 2 GB EFI partition should be adequate. After reading about BLS and system-d-boot I was originally thinking maybe only 1 GB but who knows, after typing this, maybe I will in the future want to put a dual boot (?) on this nvme which means the EFI needs more space - so maybe 2 GB EFI.
Hence having the partitioning planned now for the future is likely a good idea.
I am shaking my head at myself - the GNU/Linux world is passing me by and I feel very much out of date.