In Inkscape, layers are different from other graphics programs. That's because Inkscape strives to adhere to the SVG standards. And the SVG standards haven't created any official definition for layers yet.
So Inkscape creates layers by using special groups. Most of the time, this doesn't affect the user, and most of the time is an interesting anecdote. But I mention it, mostly just to be thorough.
Layers extend throughout the whole canvas (which is more or less infinite size, although at some point, Inkscape will crash if you make an object too large) (would be something over 40 or 50 feet, in what little experience I have with page size or object limits), so that layers can't have any particular size. In Inkscape, it's the page size that matters. If you go to Document Properties > Page tab, you can set the page size. There are many "preset" page sizes, and there are also many preset templates, which can have certain page sizes, as well.
It sounds like you probably don't need a template, although I don't really know much about your project. But you're probably looking to set a custom size.
Although if you plan to do a lot of the same kind of project, you might want to consider making yourself a custom template. So for example, you can set it so it opens at 100% zoom, with or without page border, maybe you might like a grid, already set for your custom page size, etc., etc. Pretty much anything you can do with Inkscape, can be enabled in a custom template -- just whatever you need.
Before you start, make sure you're using the current stable version of Inkscape 0.92.1, or at least no older than 0.92. That's because with 0.92, Inkscape's native DPI was changed, so that if you started drawing an image with an older version, and you wanted to work on it some more with the new version, you would have to deal with changes in the size of your drawing. But as long as you start with 0.92, you'll have no worries. (Inkscape seems to be undergoing some intense development, over this last year or so. So big changes can happen fast, sometimes.)
Also, I want to mention the page border. You probably saw it the first time you opened Inkscape, and it's probably still there, unless you disabled it. At the bottom of that same Page tab, you can enable/disable it, if you like. If you have it enabled, you might notice it change, when you change the page size.
In case you haven't realized it yet, Inkscape opens (the first time) with a zoom level of 35% (if I recall correctly). So the first page border you saw was the traditional A4 page size, except the canvas was zoomed out. You can change the zoom level to anything you want. But 100% is the level which shows the real size of your drawing.
Oh! Also note that changing the page size does not affect your drawing. If you want your drawing or individual objects to be a certain size, you need to scale them directly (Selection tool, and/or Object menu > Transform).
Almost everything I've explained here is a consequence of vector graphics. It sounds like you're probably used to using a raster graphics program. But vector graphics is a whole different paradigm, and you'll learn more and more about it, as you go.
(Sorry, I enjoy helping newbies, and I get carried away sometimes
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