What size sump???

[Carz]

So I am getting things together for my 250 gallon tank and wondering what size sump should I make. I do plan on making my own sump out of 3/8 cast acrylic and the sump will be placed in the basement so I got the room. I was thinking of something like this. Its a rough drawing but its a start.

 

[Fotocha]

Looks good