The canvas width will fit the available space. If the component output of your Joomla template expands the full width of the screen then so will the users profile and their canvas image. The height is fixed at 200px for desktop and 125px for mobile. So if you want it to be edge to edge your Joomla template needs to support being able to output components as edge to edge, but more than likely you'll need to write custom CSS to force your Joomla template to do that.

Best I can suggest for protostar is to edit it and set the Advanced parameter "Fluid Layout" to "Fluid". Then use some custom CSS to remove the padding on .container-fluid or add a negative margin. Example as follows.


There's more padding on the body element it self, but if you remove that it'll look strange being completely edge to edge. Regardless is an example that should help get you started. If you're not familiar with CSS I recommend contacting a developer to help you (we do not provide coding assistance, just simple examples). It will likely be easier to find a template that gives better control over its layout though.

The maximum width and height parameters are for resizing on upload. You have no control over the canvas size beyond that. The canvas will always be 100% width and scale accordingly to fit within the 200px height. Use your own judgement in regards to how large of an image you want to accepted based off the maximum resolutions you want to support.