I would definitely expect there to be vertical frame members at every corner, with the size of each box varying to accommodate different wall lengths. In fact, I would expect there to be a vertical frame member on each side of each corner. The alternative of chopping up the horizontals to maintain a frame size that is unrelated to the dimensions of the room would strike me as silly. This is not wallpaper.

I'd say take the artwork to a reputable framer for assistance. Often people want to use smaller frames which in fact detract from the artwork itself. Remember that great looking artwork wont always match your surroundings, but if the artwork itself looks great, it will fit in anywhere. How do you want your pictures to look? Are you going for a gallery style? Or a more formal approach? Remember to accommodate the height of the canvas frame when selecting your frame, not just the width, or your canvas will be sticking out of the framing and push the canvas off the wall, which you don't want. Personally, if you are going for a traditional look, and your canvas is over 4 feet wide, I'd want a frame that is at least 4-5" wide. If you're going to use a smaller frame, gallery style, you'll want to put a strap on the back from top to bottom to keep the pressure from bowing the framing when it's hanging on the wall. If you don't, you run the risk of popping the bottom corner joints after time.

As a professional picture framer, I applaud anyone trying to do it themselves and save a buck. However, there are somethings that most people do not take into account when using ready made products. 98% of all ready made materials you buy are not acid free and do not provide UV protection. While this may not matter with a cheap run of the mill, easy to replace poster, it is really something to think about when it comes to making frames for art that cannot be replaced. If you take a look at some of the art you have framed yourself, you will most likely notice that the bevels of the mats have turned an orange-ish brown color. This is acid seeping though the mat and onto your art. Also, the color of the mat will change and your art will fade. Even something like a family picture or your childs art that they painted in kindergarten, once it is damaged, it cannot be replaced. To me, that is priceless. If you do go to a custom framer, be sure to ask for UV protecting glass, acid free mats and make sure they do not use any adhesives that are not water soluble on your art. Art does not mean it cost you a lot of money. Art means you like it enough to put on your wall or because it has sentimental value to you. Please take these into consideration before just slapping your art into composite frames with recycled glass.

Craft store usually has them in the 8x10 size. In our area of N TX, Hobby Lobby typically has them in stock with sale prices of $2-4.00 and usually not that much higher even when not on sale. These usually consist of a sized piece of glass, a thin backing board and four spring clips which hold the whole thing together after you stack the glass, picture and backing board. Each clip is made so that there is a hole at the upper edge that will easily slide over a small nail, straight pin or whatever you usually use to hang your pictures (we typically use a stout straight pin such as used when sewing fabrics).