Should DB_RECOVERY_FILE_DEST_SIZE be smaller than the respective partition size?
10 GB as 10240 MiB?

10 GB as 10000 MB?

Is it 10 GB because of the rounding, but the actual size is 10227 MiB? Or 10213 MiB?

Setting db_recovery_file_dest_size to 10G means the size of FRA will be 10*1024*1024*1024 = 10737418240 bytes. As long as that is less than or equal to the available space in the filesystem (not partition!), it should be fine.

I usually set db_recovery_file_dest_size a bit smaller than the amount of free space in the filesystem or ASM diskgroup if I get a dedicated filesystem or diskgroup for it.