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#21
Nothing. It's a standard board size, 305 X 244 mm (12 x 9.6 inches). microATX is 244 mm square or smaller. An ATX board can have 7 card slots. mATX is limited to 4.
No overclocking? I think that some games may give enhanced framerates with overclocking, but the benefit may not be justified by the risk of instability. (You can also overclock the graphics card, most easily with software distributed by the card's maker. That has a definite effect.)
Graphics cards normally use PCI-E X16 slots. If you're sure that you will never want to use multiple graphics cards, one X16 slot is enough. No single GPU (graphics processor unit) that I know of today is adequate for gaming at 4k resolution, but I assume that you're not contemplating that.
(An aside: In the past, both nVidia and AMD have made cards with two GPUs on them. They are basically single-slot SLI [or Crossfire] setups. Usually very expensive, though. But they permitted SLI with a single slot.)
I don't know about future proofing. For gaming, the strongest dependence is probably on the graphics card. Nearly all graphics cards in desktop PCs use a PCI-E X16 slot, and I have read nothing that leads me to expect that to change any time soon. If you'd like to build a PC that you could sell in 2020 for any large fraction of what you paid new, that strikes me as unlikely, even though the rate of advance of PC technology has slowed in recent years.