How do you customize Email opens fully in Gmail?

Override `maxHtmlBytes` when you want margin. Lowering it to something like 90,000 gives you a warning band before the real cutoff, which is sensible if your templates grow steadily and you would rather hear about it early than discover a clipped campaign. Raising it makes sense only in narrow cases — an audit of internal mail where clipping is understood and accepted, or a template you know is delivered to clients that do not clip.

Stay on the published target when the result is going to be cited to anyone outside the team, since its value is that it is not your number. Keep the default for ordinary campaign checking, where the extra margin buys little and the default already leaves room. If your sends are consistently far below the limit, tightening the threshold produces findings about a problem you do not have.

Lowering the value trades quiet for early warning, and you will get failures on sends that Gmail would have delivered intact — acceptable if the team treats them as budget breaches, corrosive if they are dismissed. Raising it is the more dangerous direction, because the cutoff is not exactly where the published target sits and encoding can push a passing message over in transit. Any change also breaks comparability with previous runs, so a template that appears to have improved may only have been re-measured.

The Standard value transcribes a vendor's published guidance, so it belongs to the vendor rather than to us or to you, and it is the version worth quoting in a report. A Custom value is an internal weight budget for your templates and should be owned wherever template standards live, with the margin's reasoning recorded. Note the value in any deliverability documentation you keep, because a threshold nobody can source is the first one raised when a big campaign fails it.

A newsletter team whose digest grows through the year sets `maxHtmlBytes` to `90000`, creating a warning band so the template gets a weight pass before a real send is clipped. A brand auditing an image-heavy campaign keeps the default, because the published target is what it will cite to the agency that built it. An internal communications team auditing all-staff mail delivered to a client known not to clip raises the value, accepting that the result no longer speaks to Gmail behaviour at all.