Baffle sizes for 30 gallon sump
 

my dimension are 36-1/4"L x 12-5/8"W x 16-3/4"H.

wondering what size to make the baffles.

It's all math. First you have to figure out how much room to leave in the sump in case of pump and power failure so that whatever siphons down when the power goes off will not overflow the sump. (I installed a rely which cuts power until you reset it in case of power outage. If you lose the siphone the pump will overflow the display).

Now figure out how many gallons that would be and that will be the absolute max of the amount of water in sump. Math and measure. You will need 2 baffles at that height or less, as well as one at least 1" shorter plus one in the middle of that for the bubble trap.

Easy
(turn sideways)
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Second baffle from top must be open 2" at bottom !!!!!
 

Yep...As tall as possible while still maintaining enough room for overflow..
If you have a skimmer make surr you have a section meeting its water level or higher as you can always raise a skimmer up if its too deep