Remember--they ain't making these materials any more.
I don't think anyone has usable stocks of Portriga, and so on, these days. What materials are you missing?
My own printing seems to be without an overall standard size or format, though I did get some kit recently to make 16x20" easier to make so I've been doing some re-printing at that size to see how it looks.
Print size should be based on viewing distance. There is a formula for calculating this but I don't have it on hand. It is based on whether the entire print can be seen without moving the eye. For an intimate viewing environment an 8x10 or smaller is good. For a large exhibition room a larger size 11x14 ... 20x24 might be used.
A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.
Print size depends on a number of things. 6X7 can stand the enlargement to make big prints, but 35mm often can not. Where and how the print is to be displayed is obviously important. There are places for murals and other, quite different places for 5X7s. Small picture books can often be very effective.
I usually print 8X10s and 11X14, with 11X14 being my most frequent size for display. I sometimes make 5X7s. Lately I have made some 16X20 prints for display, and I like them. But 16X20 is about the biggest thing I can handle physically, so it becomes a limit.
I used to think that "bigger is better," until the first time that I saw Bill Schwab's wonderful little 7x7" prints. I then began to appreciate the value of small prints that invite the viewer to stand close and study them in detail.
The largest print size that satisfies my eye is 11x14 from 4x5 negatives. I do everything in my power to produce negatives so sharp that they cut your eye when you look at them through a loupe. I also like to look at tack sharp prints, which I can reliably make at 11x14. I recently made some 16x20 prints on commission. They were good, but to me, not as satisfying to look at as 11x14 prints. I also noticed that the degree of difficulty was subject to the inverse square law as the size increased.
11x14 prints matted out to 17x21 are large enough to command sufficient attention with the paintings in the gallery to which I belong, yet are small enough to encourage the viewer to get close enough to contemplate them.