This pitot tube system was patterned after one used on some vintage gliders when cloud flying was legal and encouraged. It served quite well for the small amounts of icing formed during short duration in clouds. It also inhibits impact water from entering the pitot line where it could freeze and affect the airspeed reading.
In theory, any ice which might form would coat the exposed surfaces gradually building up a thick layer. By making the entry relatively large (about 1 inch diameter), it will take quite a bit of ice to cover over it. Any water that enters the main tube may impinge on the forward side of the pick-off tube where it too might freeze. By having the open end of the pick-off tube facing to the rear, the possibility of ice covering the opening is reduced.
This is not a system intended for serious IFR as a replacement for a heated pitot tube. Rather it is a system which is superior to a straight pitot in many respects and yet is easy to construct. The dimensions are not critical.
A one inch diameter aluminum tube forms the main part. The pick-off tube can be either 3/16 or 1/4 inch. The main tube must be sealed at the end with a cap of some sort and the pick-off tube must be glued to the main tube. I had mine welded, but I think that silicone sealer would work just as well.