To many people, saying 'you can basically just work to any size you like as long as ...' or, 'a standard size is 13" x 4"' really doesn't answer all the questions they want to ask. So I'll try to help a little with this.
Perhaps Lee Nordling's book, Your Career in the Comics is still the best guide. Here's a page or two from it:
Still unsure? Okay, I've highlighted the big half-page format with an X for a reason. That is the daddy of the Sunday foramts and it can be aranged in a number of ways. If you read Lee Nordling's book you'll see that it is a very clever and intricate piece of work that can be converted into a number of different formats. The most obvious and most frequent change to this template is that the newspaper can dispense with the top row, panels 1 and 2 (which is why it is often a throwaway gag or maybe just a comment, and the strip still reads as a complete strip.
These sizes are sizes you can either work to, or scale-up from, so long as your work scales back down to these sizes (I'm reluctant to put, or thereabouts).
I think we can make this a little easier by using a Peanuts Sunday template, full scale. The original was larger than this, Schulz worked larger, but the copy for publishers fits the dimensions of Format A, above. Remember, you can scale-up (click for the hi-res copy)
It still smacks a little of the dark arts, I feel, and especially so to us UK cartoonists who could not, you may recall, add our own tone to drawings until the 1980s, when we often still had to submit original drawings, and so everything, literally, was done for us.
Templates then, are very handy. However, I have a few Manga templates - blue-lined pages (blue does not scan or photocopy on certain settings) that are finished drawing size, the same size as the thing is published, rather than the size the project can be comfortably drawn at (see my post on the difficulty of drawing the Bee-Man same-size).
Blue-line Pro pads look pretty good. Those are Bristol Board sheets with 2 daily strips, 13"x4" and 1 Sunday strip ready ruled, in blue line.
And you can now add the Canson Fan Boy range to the mix. My friend, cartoonist and writer, Paul Abittabile (who helped with the Canson Fan Boy design) sent me the latest Canson Fan Boy catalogue and I feel a bit like a kid in a sweety shop, albeit looking in fropm a distance as, yet again, I'm going to have to get the tools I need from overseas. Like a lot of illustrators I'm familiar with Canson's products, in fact Canson is one of the few Bristol Boards you can buy in the UK, so I'm really looking forward to working on these. Anyway, it looks like a well-thought out range, including layout pads, comic book pages, and comic strip and even Manga-page formats, and I'm certain it'll go some way to putting the minds of we mathematically challenged cartoonists at ease. Perhaps now, size won't matter too much.