plumbing from overflow to sump

I am starting to plan out my plumbing design, and i would like to start this thread to start my questions as i go along. I have never plumbed anything so this will be interesting. i hope it looks good when done, but we shall see.

i have two overflows in my 135g tank. in the overflows i have a drain line and a supply line. when buying a coupler that will screw into the overflow lines......should i keep the plumbing lines that will lead to the pump/sump the same size as the lines in the overflow, or can/should they be increased/decreased?

i hope i am making sense here
 
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