Bag of Holding Dimensions (2024 DMG)

In the 2024 DMG and on line, the dimensions of the Bag of Holding reads as: "dimensions—roughly 2 feet square and 4 feet deep on the inside. The bag can hold up to 500 pounds, not exceeding a volume of 64 cubic feet."

If the ‘inside' extradimensional space dimensions are 2-feet by 2-feet by 4-feet, as stated, that is a total volume of 16 cubic feet, not 64 cubic feet, which would be 4-feet by 4-feet by 4-feet.

So is the internal space 16 cubic feet or 64 cubic feet?

The 2 ft square is the opening. The 4 ft deep is the inside. The inside is shapeless - like a sack - so would be 4 ft by 4 ft by 4 ft. 4x4x4 = 64. The interior is 64 cubic ft in volume.

You can also just ignore the internal measurements and go off weight alone. Notice said "internal measurements" because the bag's opening is still defined, and some DMs will hold to that.

If we were going to take the bag at its word, then fitting things like staves, spears, halberds and many types of greatsword in the bag would be impossible, since they exceed 4 feet long. 
The Typical Zweihander ( approx 7 feet), Montante ( between 6.5 and 7 feet), or Oodachi ( approx 7.5 feet) are all longer than 4 feet, a quarterstaff is often historically 6 to 9 feet, but do we really want to open the can of worms of  "That is too long to put in your bag of holding." 

Heck sometimes the opening is ignored. A lot of DM will let you store shields that should be too big to fit in a 2 foot opening in them with no problem. 
When i am running games i treat it as almost cartoon hammerspace and will make on the spot rulings of when something doesn't fit. Does lead to a lot of:
"No you can't put the adult dragon in the bag of holding." 
"No you can not put the storm giant in your bag of holding and give her a piggyback ride. But creative thinking."
"No you can't put the church in the bag of holding."