UEFI standard requires a FAT32 partition. It is usually a first partition on disk, 500MB in size. The linux kernel to boot, together with initrams disk can be placed in the root partition, usually ext4 or btrfs, or it can be placed in uefi partition. Limine will only support booting the kernel from uefi fat32 partition to avoid having to support ext4 and btrfs filesystems. The root partition will still be ext4 or btrfs, the code to mount it would be in kernel or in initramfs.
Edit: size of uefi partition
I am using zero for drone fpv running openhd. And a rp2040 based board to control rc truck. I also have odroid c2 controlling a voron v0.2 3d printer.