Check this link out to the DIY forum:
This is a combination Sump/fuge and was really easy to do. I built this using a 30g tank and it works great. Basically the fuge is on the far end of the sump and water returning to the tank is diverted to it via a T in the line and a ball valve to turn down the flow rate. Very cost effective compared to many inferior pre-made products out there.
You can customize the measurements to fit your equipment. Use any size tank you desire to use as your sump/fuge. I went with a 30g because it was the biggest I could fit under my 90g.
As for size my suggestion is go with as big as you can afford/fit. The larger thew sump the more water volume you will have which is always better IMO
If you have a standard 125 I would think you could fit at least a 55g under it no problem. If you plan is to install the sump under your main just verify that it will fit without tearing down the main. My original plan was to use a rubermaid tub temporarily until I had time to build the permanent sump I wanted. Fortunately 3 days prior to pick up I realized I could not fit a 30g tank under my 90 through the door, so I built the sump early and installed it at the time of set up.