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If you take the SNES as an example:
256×239 native resolution

If using a 1080p screen you would have a max of 1920 x 1080

So in order that the height not lose native aspect ration = 1080 / 239 = 4.51(plus rounding).

So height is 4.51 x 239 = 1078 (rounded)
Using same multiplier to keep aspect ratio for width would be 4.51 x 256 = 1154

So SNES could be 1154 x 1078

Although I know you have suggested 1194×896 for SNES.

So why might you not want to use the calculation above? Because as far as I can see it maintains the native resolution as well as maximises screen area.

Or are you saying because most CRTs would have forced an effective 4:3 view, you want to maintain that aspect ratio?