Now, no offense, but you're completely missing what responsive layout, or even modern layout concepts are even about. The whole IDEA is to re-arrange to best fit the available space, creating a "fixed" ANYTHING, be it width, positioning, or even height, is the antithesis of that and bad web design. A decade ago I was calling fixed width layouts "crappy little stripes" -- and to be frank, the concept of fixed layouts has only gotten worse and less useful as time passes.
Even THINKING about a single appearance across all devices is broken thinking and garbage methodology -- and why a lot of common practices like crapping out a PSD and having the giant set of brass to call it a layout (and the people who do so having an even bigger pair calling themselves "designers") is effectively putting the cart before the horse, and is a completely back-assward approach to developing a website.
If you are aghast at the idea of your content re-arranging itself to fit the screen, or things oh noes scrolling off the screen, you may want to consider doing something other than building a website. That's what they're supposed to do, and the handful of sites that don't are buggy unusable inaccessible train wreck laundry-lists of how not to build a website.
How much have you learned on document structuring and semantics? Not a whole lot I imagine since you mentioned HTML 5. There's a process to building a website, and the most important question is: "What's your CONTENT?"
The most important thing (despite your typical PSD jockeys claiming otherwise) is your CONTENT, as it should be dictating your markup, and the content combined with the capabilities of each different user-agent (browser) and device should dictate the layot.
You start with your content in a flat text editor, and mark it up semantically in a RECOMMENDATION doctype. Do yourself a HUGE favor and forget all the pointlessly redundant code bloated nimrod bekaptah allegedly semantic new tags in HTML 5, and use PROPER orders of headings to provide structure and normal semantic markup...
Proper orders being that the H1 is the heading under which everything on the page is a subsection, which is why the best candidate for the H1 is the site title in most cases. (no matter what the black hat SEO scam artists try to tell you) -- just like a newspaper or a book; while the presentation is different on the home page, what's at the top of EVERY page of the paper? The name of the paper. H2 are the start of subsections of the H1, H3 are the start of subsections of the H2 and so forth -- which is why skipping over heading levels (h5 after a h2 for example) is gibberish... and a LOT of developers miss that semantically a horizontal rule (HR) is a change in topic where a heading is unwanted/unwarranted -- just like in professional writing. This of course is why HTML 5's idiotic "SECTION, ARTICLE, NAV and FOOTER" are redundant code bloat that serve no legitimate purpose apart from the same data-scraping idiocy as "microformats" before it.
Really though that's what semantic markup is, GRAMMAR. If you aren't using the tags like they were grammar... well... as I've said several thousand times the past decade:
"If you choose your tags based on their default appearance, you are choosing the wrong tags for all the wrong reasons."
Logical heading orders, paragraphs around grammatical paragraphs (not images, not fragments, GRAMMATICAL PARAGRAPHS), UL/OL around lists of short items (if it's big enough to warrant a header, it's not a list), tables around tabular data (and not just to make columns), etc, etc...
That content and markup combined with the size and cababilites of supported devices then dictates your CSS layoutS -- PLURAL because you shouldn't be making any single fixed layout. The entire reason we have a MEDIA attribute on LINK to target specific media types, and media queries to build responsive layouts is the idea that one fixed design CANNOT meet the needs of all users or devices. That's the POINT!
... and yes, that can make it very hard to do; more so if you try to do things that to be frank, have no business on websites in the first place -- like 99% of the garbage people vomit onto websites with "frameworks" like jQuery, "grids", bloated garbage like bootstrap, etc, etc.
Bottom line, anything you make "fixed", even if you scale it size-wise to the device, is going to piss off a user somewhere at some time. Anything you make for the smallest of smartphones at 192x240 is going to piss off a user on a desktop like mine at 2560x1440 and vice-versa. It's why "fixed" design should have gone the way of the dodo fifteen years ago.
No matter how many PSD jockeys and people still sleazing out HTML 3.2 with 4 tranny or 5 lip-service around it might try to tell you otherwise.