I would think first thing to decide is for what purpose and where are you mainly going to be using it. If you plan on only having it as an emergency signaling device that you will pretty much blow up on surface, I would get something very visible and large that is closed circuit, and easy to hang onto.
However, if you plan to (also) use it as a surface marker buoy often, ie shoot it up from depth to mark your position there are maybe a few more things to take into account. Ease of use becomes more of an issue. It is not most comfortable and handy, say on drift dives, to handle very large SMBs, and in some environments you do not really need to be THAT visible especially if it’s not emergency. If that was the main purpose, smaller one would be way more desirable for most cases.
Also think where you will be carrying it. The bigger the more space it takes. Maybe you do not need a huge one on some quarry/lake dives. I personally carry small (3ft), orally inflatable one for lake/river/quarry dives where I use it to just indicate my position. In ocean I carry 6 ft “Big Frank” in addition to the small one.