A Class-4 SDcard for absolute minimum, a Class-10 for comfort. For better read and write speeds, use a SSD via a USB3.0/SATA for booting and running LibreELEC.
RPi 4 with 2GB is good enough for most (all?) users.
I'd go with 4GB just in case (price difference isn't humongous) but 8GB seems to be overkill.
Edit: USB3 to SATA adapter and a decent speed (small) SSD as RPi4 supports USB boot and You are good to go for quite a while...
what about the storage minimum
My current average Kodi folder is some 3GB, including the thumbs cache folder which is the biggest part.
Since you are also using a NAS, things should be roughly the same unless you have a pretty big media collection.
So 4GB and upwards is fine.
RPi4 2GB model and up are fine for LE(10) .. in our testing the 1GB model is fine for 1080p media but has issues with 4K.
SD cards 4GB sized and up, but more than 16GB is probably a waste of money as big cards are sill prices so you're better off with a small(er) boot card and media on a USB drive. The smallest cheap cards I can find in the shops these days are 16GB class 10 cards.
I have a external SSD USB; Netac 256 GB USB SSD. But the LibreELEC USB-SD Creator (v 1.4) does not recognize it as an USB stick.. Any idea's how to fix this? I tried formatting the USB SSD to NTFS, FAT32, EXT4, but that does not seem to help...
I'd guess the USB drive is presented to the OS as a hard disk and not a removable storage device (flash stick). The creator app is hardcoded to hide hard drives to stop users overwriting something important. Perhaps try other apps like Win32DiskImager or Etcher?
