@MALOJFK there is a bit pit in front of one of them, and you can get a block without leaving a hole in the ground.
I would probably put them between 2.5-3 blocks tall. They are certainly taller then you. I would say keep your home 3 blocks high so they can't get in, but if they can take it apart that becomes meaningless.
They probably won't take apart your house without a loophole to avoid them. Maybe one spawns every night, so if you don't kill them they pile up. it seems like they probably only can carry one block at a time, so if you keep them in check it should be fine.
Also, might want to make walls with more then one layer.........just in case.
So it seems they are a night time only mob. They also seem to spawn in treeless environments. They are able to break and take sand and dirt blocks. Its assumed they will be able to destruct/construct blocks and buildings. I agree with Sneaky Chicken and this mob is something you should keep your eye on. But its unknown if they die in sunlight as spiders and creepers don't.
what you probabally didnt realize is there seem to be two different kinds of the "slendermen" some are 4 legged and some look like they have one... they seem to build tho because one has sand and one has dirt with grass on it... my question is why does it have grass on it? do they build do they make the mushrooms? please comment.
If they destroy houses, you could make it with 3 layers. 1st= nonflammable block. 2nd= lava. 3rd= nonflammable block. If it tries to destroy your house, it gets burned to death! that will keep em off your lawn.