They need to be exactly twice as long as wide in order to stack correctly.
Generally the depth would be 4/5 the width, so that 5 on their sides is the same as the length of two.
The width and depth are fixed by the baler, so it's not easy to vary the length of the bale without losing those ratios and getting bales that won't stack safely.
However, they can be baled more or less dense. If densely packed, they'll be heavy to move, stack, etc, but you're getting more for your money. If less densely packed, they're easier to handle now, but come winter, the strings will be too loose and you'll find them coming apart in your hands.
Not sure if that helped any...
Size isn't the best measure, really. We work on 40 small bales per tonne. But unless you have a weighbridge you can get your load to, that doesn't help much either. Sorry.