Here's a few I made earlier.
They are various sizes (the smallest one at the front is 8 inches square) as currently I am canabilizing old canvases to get the stretcher bars. However, I am trying to gradually move over to a few standard sizes - 10x12, 12x18, 16x20, 20x24 inches. A gradual process - and a little scary, those bigger ones are bigger than I've painted so far.
However, one of the advantages of stretching your own canvases is that you can decide the proportions yourself. Another is that you can control what kind of surface you have.
So, what do I use? Previously, unprimed fine linen from Jackson's Art but when I was in London I visited Russell and Chapple and am now about to use their portrait linen - it is really lovely and very much recommend a visit to their shop to see their range.
Once stretched, I use GAC 400 rather than rabbit skin glue, then GAC 100 to prevent the oil sinking in, then finally Roberson's Oil Primer. which is just amazing (if smelly) stuff. Unlike acrylic gesso it provides a slippery surface that really lets you push paint around and as it doesn't self level you can create a variety of surfaces - from rough to very smooth. I go for the fairly smooth, using 2 or 3 layers, each one applied with brush, then unified with a roller and then finally sanded, except the last layer. This gives me a fairly uniform surface with a slight "fuzz" to it that I feel is sympathetic to painting flesh - if I was wanting to paint seascapes, I'd maybe leave the brushmarks.
So what do you do?