how does the terrain compare to F3? are the mountains bigger/smaller?

There's a lot more actual hills. The hills in FO3 just kinda rose gradually, but these can be downright steep.

Some of the middle-of-nowhere mountains are pretty annoying, I have to admit. Slopes that look climbable are not climbable, mobs get caught in the rocks, and your character stands in mid-air about 3 feet above certain terrain.

I'm not really bothered by it though, because this only seems to happen waaaaay out in the nether areas where there is no real content.

Heh, true.

But at least you can actually climb things now. Remember how FO3 would ignore your jump "request" if you were on so much as a pebble? Not an issue now