I looked up to see if anyone else had brought this up, and no one had. I had to literally go on one of my posts to make sure my sig wasn't hilariously oversized after changing it.
Hi @Magnezone - this is indeed not a feature available when editing your signature. An easy alternative I would recommend is using the post editor to compose your signature, as if you were going to open a new discussion or comment on one. This editor does have a preview option, and once you are happy with the result, you can copy and paste the text into the signature editor
Comments