It all depends on how many, and how big the fish you are planning on putting in your tank! I have a 90g tank with a 20g sump, but my rock takes up about 25g water space so now I'm looking ate about 75g, and I use a reef octopus skimmer rated at 300g because I have very large fish in my tank, and also I have a lot of them! I know peeps say you can't do it, but Ive done it for like 6 months and I run a full reef tank, sps, lps, and softies!
Oh, also I run a phosphate reactor rated for a 250g tank, and another reactor for a 150g tank so thats 400g's of reacting! I call it over reacting!!!
One thing though, if you are over skimming, and over reacting, you must add your trace elements, and amino acids, consistently, along with 30% or more on your water changes!!!!
Good Luck!