I have laced for the top (snaffle) rein and plain for the bottom (curb) rein. The curb rein is slightly thinner than standard (mine are 5/8 for the laced and I think either 1/2 or 3/8 for the curb). Length depends on how big your horse is - mine are normal full length. Just make sure you don't have too big of a loop hanging down.
The above would be a fairly standard setup, but equally some prefer all plain, some use plaited for the top rein, some like the half rubber ones (plain on the outside, rubber on the inside) - so just go with whatever you prefer!
