Looks like Euphorbia lactea, maybe. There are a few species in the ballpark. You can tell E. trigona (quite common) from E. lactea (less common) based on how they branch. Check out the plants on those two pages (Lin left you the links) to see what I mean, but basically trigona branches are all about the vertical, right next to the mother stem, and lactea branches are much more sideways, even if they eventually become vertical.
All of those plants have the potential to get quite large given time and space.
They like excellent drainage, regular water when the soil is dry, and strong light if kept indoors. Protect from winter cold and use the bare minimum if you decide to feed.