Pecan trees are very large, I was going to guess 100 ft, but I found this on the 'net **The pecan tree is a large deciduous tree, growing to 20�40 m (66�130 ft) **
And I am guessing the width of the crown is almost as wide! My sister has a lot of pecan trees near the Gulf Coast, and limbs fall fairly often esp. in storms. I would definitely locate the tree far from the house.

Pecans are notorious self-pruners ... even on clear, sunny days. Plant far away from any structure.