Hello @HalimaR,
The average pixel width of the header is 600 pixels, for all of the standard 'basic' templates. That's true of most of the other standard templates as well, though if you search for anything with the 'wide' tag those are 800 pixels. The height does not matter, as the block's height resizes automatically based off of the content. There are a few other exceptions, but that should be the same across the vast majority. Every image in the new editor is meant to be easily re-sizable though, so click the two-pointed arrow in the bottom corner of it, and dragging downwards, should make essentially 'any' image full-size, without significant distortion. The new editor is better at resizing images than the legacy editor, though it's admittedly less precise.
We are always looking to implement further upgrades to the system. Some additions, like html access, have other implications which cause delays, or prevent us from enabling them easily for all customers. We won't release an update if we fully believe it will cause more harm than good, but we do take your feedback very seriously. I can honestly say that internally there's a lot of discussion throughout all levels of the organization about how best to give you the tools you want and need, while keeping the system as stable and reliable as possible.
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