I'm one of those people who just posts regular comic-pages (I'm using Tapastic as a mirror for my main site), but one way you COULD do it is to draw each panel individually, and upload them one by one in the same episode - each episode allows 40 images. The one you upload first will be at the top of the posted page, and so on. That way, you don't have to do one giantically long vertical document in Manga Studio - you can instead do individual panels (or sets of panels) and just place them below each other when you upload.
Also, that means Manga Studio doesn't have to work super-hard to save your giantic file, which is good.
Panel size, of course, varies depending on what you want to draw in them. Tapastic has a 940-pixel limit on width, so do 'em a little smaller than that, with a white border, is my recommendation.
As for font-size - like I said, I do regular comic-pages, and letter them with 12-13 pt font, and it's just barely readable on the screen of my ancient HTC Desire, which is, what, 3 inches wide? More modern phones (and tablets) will have larger screens, and therefore larger display-sizes - but yeah. 12pt font should be your lower limit; going below that makes the text really hard to read on mobile devices.
As for general tips on the vertical format - keep in mind that, unlike a regular comic-page, people aren't going to see your entire "page" at once; they'll be scrolling down to see the next panel, and then the next, and so on. So if you draw something, you can't rely on the reader simultaneously looking at a previous panel the same way you can with a regular comic-page, which is usually viewed as a unit, even if it's read panel by panel.