Some braces are adjustable (SB PDW, SBa3, SBa4) without tools, some with tools (KAK Shockwave blade) Some not. Well, I guess most of those are too, just don't cram them all the way down the tube. My personal experiences are with the SBa3, the SB Mini, and the SBM4. The latter two are non-adjusatable models. I first bought the Mini for a 7" CDNN kit, and it was OK. I used the foam collar that comes with the kit to fill up the bare tube area, though I had to modify it to clear the charging handle. I replaced that with the SBM4 when CDNN had them on sale for 1/2 price. The SBM4 was more solid, though bulkier. My next build was a Palmetto 10.5" kit that came with the SBa3 brace; I really like it. I suspect I would also like the SBa4. As you can guess, I'm not a huge fan of KAK's, but some like them. You set the length on them with a set screw. And, as I mentioned, the non-adjustable ones, you just don't seat them all the way down the tube. The SB mini is about the only on were you really don't have that option, as it is really short.
The mini on the 7":
The SBM4 on the 7", my son shooting it:
The SBa3 on the 10.5" PSA:
So there you have my experiences and opinions concerning AR pistol braces. HTH.
Forgot to add:
https://www.sb-tactical.com/
https://www.kakindustry.com/
Start looking for your new brace!
John Lovell has a good video on braces, but he does misspeak at 1:53, (about the legalities) and I called him out on it in the comments. I also mistyped SBa4 when I meant SBM4 in my reply.