what size SUMP & what to put in it

what size SUMP would i need for a 8oolt tank? was thinkin maybe 48x18x18 ?

would it be better to have 4 compartments or 3?

whats the best things to put in the compartments eg 1 2 3 4?

should i go for a dsb with cheato in it or Miracle mud? want somethink that will be help with Nitrate, iv got 2x reef fresh h20 blocks that i can put in a compartment.

 

There is no general sump size for a tank but the rule is "as big as you can go".The more water volume the better.The "norm" is usually 3 compartments,first would be your intake from your tank where the skimmer usually goes,the other two can be how you want it,some people go with the return in the second compartment with the third being your fuge.
Here's a good link that should help...
http://www.melevsreef.com/make_a_sump.html
I have sand in my fuge and have no experience with the mud so no help with that(sorry).

 

You are actually referring to a refugium, since you are talking cheato and DSB. Since you will have a combination sump/fuge, make sure you have proper flow into each compartment.