Hi - there's no width maximum being enforced by Marketo itself. (I have some over 1000px wide).
I would think this is probably an HTML issue. Is there any chance you are attempting to change the width of a nested (inside) table rather than the outermost table?
If you check in the HTML, you want to check the very first table tag after the <body> tag. It'll be <table width="xx"> (There may be other values in this tag as well). Whatever that value is in that first tag is driving the container your email sits within.
That's my best piece of advice without seeing the actual code behind the template.
Veronica
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