Rebuilding porch stairs - what size for treads?
You can use 2 pcs of 5/4X6" to replace each tread. It will keep the same dimension as your originals, 5 1/2" + 5 1/2" + 1/4" gap = 11 1/4" This is a very typical detail. You should have an overhand at the front corner, call a "nose". Your originals may already have that, if so, simply follow the same detail, but using the two pieces per tread.

Edit added sketch of treads, etc.