Gmail signature too long
Google documents a 10,000-character Gmail signature limit and notes that signature images count toward that limit. Generated HTML can reach the limit surprisingly quickly because inline styles, repeated table markup and decorative elements all contribute to the source.
Primary Google evidence
Google’s Gmail Help documentation currently states that signatures can contain up to 10,000 characters.
Read the Google Gmail Help source →
How to reduce signature size
Start by removing unused wrappers, duplicate inline styles, hidden content and unnecessary decorative markup. Keep important contact information and branding, but prefer simple table-based structure over layers of generated layout code.
What this rule does not mean
This is a Gmail signature-setting limit, not a general certificate that shorter HTML will render identically everywhere. Outlook, mobile clients and other mail applications have different compatibility concerns, so use the full checker when the same signature will be installed across clients.