There isn’t any way to know what a cubit was unless the person that did the measurements left behind some artifact that we can measure now. Sure, we can all read the numeric specifications in the Bible. Unless there is some way to have an equivalent in our measure, we cannot look at anything and say, “Look! That is exactly 300 cubits long”. As it now stands, I could declare the Empire State Building to be exactly 300 cubits tall while you could declare the length of a Viking longboat to be exactly 300 cubits long.
The cubit is still used in the garment industry. Because the distance between the elbow and the middle finger varies with each person, cubits are used to design some garments so they will fit an individual.
Whatever the cubit length was it was 300 of them.