I will install RHEL to a server ( for database storage ) , I know we can divide the hardisk into partition by size or do not specify the size , if do not specify the size , then the partition will be as below .

Filesystem 1K-blocks Used Available Use% Mounted on/dev/mapper/VolGroup00-LogVol00
73545144 50176316 19572656 72% /
/dev/sda1 101086 12041 83826 13% /boot
tmpfs 481884 0 481884 0% /dev/shm



I just want to ask which is better , I guess the second one is better as the server is less chance to reach the size limit and it is more flexiable to change in future , am I right ? which one should be use ?

Thanks

I don't see a big issue with your partitioning.
I believe you have a 80GB disk, right?
If so, then a / partition taking up all available space is fine.

If you were on a 500GB disk, I would advise 100GB for / and the rest for /home. This way you could backup independently the OS and your datas.

It's always a bit hard to advise with little infos on your config and your needs