How much powdered milk and water would you need to make exactly 5 tablespoons of Milk?
Five tablespoons is a smidge under 1/3 of a cup. There are four tablespoons in a quarter cup.

You don't say what the reconstitution percentages are for your powdered milk but if you do whichever conversion is for 1/3 of a cup, you should be fine. Most recipes don't need it to be absolutely exact. A little extra milk isn't going to harm anything.

One website says to use 1 tablespoon of powdered milk and 1/3 cup of water but this other one says 2 tablespoons and a teaspoon with 1/3 cup of water... so it may depend on the brand.

Please note, depending on what you're using this for, you may need to add fat of some sort unless you have whole milk powder, as many powdered milk products are non-fat. Most recipes that call for "milk" mean "whole milk" substituting fat free milk (skim) can be detrimental to the final flavor of the milk.