Sump questions

[inperio]

I have a 90 gallon standard 5ft tank.
I'm planning to convert it into a saltwater aquarium set-up.
standard 90 gallon double stand
I have a skimmer rated for almost 300 gallons. which will take up about a foot in length.

Now I need to have a sump made. What are the measurements? Should I make the sump 5ft as well or smaller? How many chambers do I need? should I put the skimmer inside the sump or outside?

Does anyone have standard measurements for a sump?

Thanx for the help.