are you doing this in the code or the editor?
The doc for the editable elements in email templates is here: Email Template Syntax - Marketo Docs - Product Docs
More specifically, the mktoImg element has some properties that enable the developer to lock sizes and styles.
Please also be aware that in most email templates, the size of a given elements might be set mutlple times (unlike what you would be taught in dev school), because the various email clients (outlook, apple mobile devices, android devices, ...) will not take the same sizing instructions into account. The result is that you may think you have changed in, but a media query may overcharge it on certains devices (and in Marketo preview, that supports media queries, unlike android or outlook).