Most commercial sumps rated for 125G will only be from 20-25 gallons. With that said I would get as big as sump as possible. I have seen plenty of setups with the sump just slightly smaller than the show tank.
...The 55G tank seems to be a very common and inexpensive tank. Especially if you get one used.
If it were me, I would start looking at 55G and go up from there. The more water you have circulating in the system, the more buffer action you will get out of it and the happier fish you will have. :D :D