I'd like to share an idea why you're getting that message, dafyddparry.
You need to consider this file types to avoid issues in uploading your custom form style.
- Audio
- ClipArt, Word Art
- Animated GIFS
- Embedded files
- Adding or updating hidden links or merging fields
- Files over 2MB
- Non-.docx file types or older MS Word documents renamed as docx files.
The error message you get is the size validation error. In order to fix this, make sure that your document is less than 2MB. You might want to create another import style then try to download a sample document to check if you still get the same error message.
You can check this detailed articles for more information: Import custom form styles for invoices or estimates.
Let me know if you have additional concerns with importing custom style, I'd be happy to help.