Chosing a pulldown resistor
10 k&Omega; is a typical value for a pullup/pulldown, and the 500 &micro;A loss at 5 V is usually not a problem; the optocoupler's LED will usually need a multiple of that. For low power applications you can increase the value, and the upper limit is determined by the 74HC244's input leakage current. The datasheet says that is maximum 1 &micro;A, then a 1 M&Omega; resistor may cause a 1 V drop across it. That's a rather large value, still OK at 5 V supply voltage, but I would choose a lower value.

A 100 k&Omega; resistor will cause a 50 &micro;A leakage from the device's output, and a maximum input voltage of 100 mV when floating. This looks like a good solution.