when I create Web Users under a client
am I able to limit disk space & bandwidth of those Web Users?

no, you can't do that

Yes, you can but you need to know how hard quotas work.

Plus the quotas will need to be turned on in your linux box.

Once you create a web user you then run the command in the shell prompt

/usr/sbin/edquota username

That will create a hard quota they can not go over with.

I have mention this to plesk several times in the past to create this in thier new releases but they just dont.