@Nebulaz, I will try to explain as best as I can below.
Hi everyone. I'm about to set up 2 AX86U for the first time. I will be setting them up in AiMesh. Any advice on how to do this? If you have a link to a topic or steps posted elsewhere I'd appreciate it. Also, should I immediately upgrade the firmware on each to Merlin's beta? Or should I first...
www.snbforums.com
As to the 'why', it is/was an issue that some posters had come across when using short SSID's and/or anything other than alphanumeric characters, no spaces, etc. see link above. If memory serves me well, it was within the last couple of years on these forums.
I arbitrarily started using 8 characters (for no other reason than a nod to old DOS naming limitations...) for all my customer installs and found improvements for them vs. what I could have expected previously. For reference, I have always defaulted to 8 characters myself. But when others found issues with short SSIDs, I made it part of my 'best practice' steps that I deliver for each customer.
While I wouldn't go shorter than 8 characters for all the reasons above and more (almost anything can be described in 8 characters, with some imagination), there is no reason it can't be longer. Up to a maximum of 16 characters is the most I would suggest. Asus has arbitrary limits (16 characters is the limit for the Username, for example) that are not obviously, transparent, or easily linked to issues expect by fortuitous experimentation (like the short SSID name proved).
Because I like to keep things consistent for each customer, I do not like to give ranges (8-16). That is why 8 alphanumeric characters, with no spaces, punctuation, special characters, or smiley faces is my recommendation.