The sign size model is actually a 24x12 px when in game is actually gonna be a 16x8 px. It result in pixels not matching when side to side to other textures. Trying to change it with the help of custom model and optifine will result with smaller sign and there is nothing possible to do.
Thank you for your report!
However, we are already tracking this issue, and this report is a Duplicate of MC-195473, which has been marked as Works as Intended - this is an intentional game feature. If you have a different opinion, please visit the Feedback website.
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