Overflow Size?

[jer77]

First of all, what is linear overflow size? And for a Mag12, what linear over size and drain diameter does it need.

[The Punisher]

Linear overflow refers to the linear length of your overflow (the slotted teeth). The linear overflow length and drain size your pump will depend on how much flow you are getting out of it, which will depend on your plumbing and how high your pump has to pump water up to your tank.
Check out the head loss calculator and determine how much flow you can expect:
http://www.reefcentral.com/calc/hlc2.php
And then check out the drain size/overflow calculator and plug in your GPH:
http://www.reefcentral.com/calc/drain.php
This should give you a good idea of how to proceed.