External Pump and Sump size

I am moving to a new home. I have a 100 gal tank (4ft long) converted as sump. I won't have enough space for it in new home. I have a 120 gal display tank.

1) What sump size is good for 120 gallons?
2) I want to use external pump for return. I will be using 2 mp40s as well. What brand and size should I get?

Thank you
 
I just replaced my external dart with a waveline dc pump and it's awesome. Adjustable rate, feed mode, soft start, and being internal you don't hear it at all. It can be tan external too.
 
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