The maximum size per file is 10 MB. That ceiling applies everywhere you attach something: images and documents in the demand wizard, per-item attachments, offer documents and company profile images.
The limit is per file, not a total allowance — you can keep adding files under 10 MB as long as your plan's attachment count allows it. The hint under each upload area repeats it: PNG, JPG or WEBP · up to 10 MB (for documents, PDF · up to 10 MB). Pick an oversized file and it is skipped with File is too large (max 10 MB), while the other files in the same selection still upload.
Your plan may set a lower per-file limit. The figure that applies to you is shown under Plan limits on Settings › Usage, on the File size card. On the Free plan it is below 10 MB, and a file over that plan cap returns File is too large for this plan.
If a file is over the limit, resize and re-save the image, or compress the PDF and try again.
Also asked as: What is the file size limit? · How many MB can I upload? · Maximum upload size? · How large a PDF can I attach?