Volume-wise, it's always going to depend on the diameter of the bag. Bear in mind you can always roll the top down further to reduce a bag's width between the bars.
My Alpkit drybag came with straps, but I find them bloody fiddly to use, and have replaced them with some basic luggage straps.
Depending on the size and weight of your load, you can strap a drybag directly to your bars, so a harness like the Joey isn't essential.
Volume-wise, it's always going to depend on the diameter of the bag. Bear in mind you can always roll the top down further to reduce a bag's width between the bars.
My Alpkit drybag came with straps, but I find them bloody fiddly to use, and have replaced them with some basic luggage straps.
Depending on the size and weight of your load, you can strap a drybag directly to your bars, so a harness like the Joey isn't essential.