There is a LOT to consider in stair construction, at least for <i>real</i> stairs. But the very basic typical measurement is that each step goes up about 7 inches and forward about 11 inches.
So in HO scale that's .08" X .13", or 5/64" X 1/8", or 2mm X 3.3mm, depending on what measuring tools you have at your disposal
Central Valley has a package of ladders and stairs in plastic. The stairs are around 4 inches long and you can shorten them to whatever size you need. If you prefer wood then Northeast Scale Lumber has pre cut stringers and you add the treads from strip wood.
I recently built a laser kit that included peel and stick details and the stairs were made that way. The contact patch of the sticky treads onto the stringers is so small the finshed assembly was as fragile as a spider web. Ended up strengthening the joints with thick CA. In the future I'll just use some pre mades from Central Valley and use the peel and stick parts for kindling.