I'm wondering why this question is coming up, Since most Hex keys come in sets.
May I ask why this information is needed?
You use some more than others.
For instance, I have a full set of t-handles in SAE from 3/32" to 1/4" and in metric from 2.5mm to 10mm. Those are handy for shop work, I don't ever take them unless I know I will need them, or I will only need one or two, they are too bulky to carry more than a few.
It also makes sense to have more/better ones in the sizes you use most, I have 3 different styles of
t-handle 5/16 hex keys, one is smaller and more
portable, I are longer and have different comfort handles. All three are of much higher quality than my SAE and Metric sets because they get used the most and I can't afford for them to slip or
break in the middle of something. I also tend to put them in different "kits" so that I always at least have a 5/16".
Then you get into hex key jack-knives, much more
portable but not as strong. Certainly good for quick repairs, but you wouldn't want one to put together a large
deck. Plus they are just too short for some things. I thin my biggest is 80mm long.
The there are hex drivers that are great for assembly and okay for a
t-handle substitute in most cases, but again, you don't get as much torque behind them as a
t-handle. Of course usually if you need a 300mm or longer hex size it's easier to find in a
driver verses a
t-handle (excluding the "
deck stick").
To the OP more directly: if you haven't needed them yet, may I suggest that you hold off on it until you fill out some of the more basic tools, unless you are done there. The only size I use frequently enough to always carry is, as mentioned, a 5/16"
t-handle, and a decent jack-knife. Other than that, you'll know when you need something.