Need help designing a sump and plumbing!

[HeckYeck]

Hey guys! I have decided to make my first attempt at a reef tank and picked up this cute little 12g at my local aquarium store, I am planning on using one of the many 5.5g's I have lying around as a sump and want to drill holes in the back of the display tank for that. I have drilled holes in a tank about 10 years ago to make one that would have high and low tide for a group of mudskippers but I have never made a proper sump before. My question is how big does the overflow need to be in order to allow for enough flow as to not overfill the display tank? If it helps my pump flow rate is 155g/h (but adjustable which I am assuming I will need to take advantage of) and uses 1/2" tubing. Additionally if anyone has any ideas or just encouragement regarding lighting a tank this long I would love it lol, I feel like I'm really going out of my comfort zone in many ways with this one but very excited! Thank you so much for any help!!