Determine curtain width, length and fabric needed for any window.
Curtains are sized from the rod, not the window. The rod normally extends about 8 inches past each side of the glass so the panels stack clear of the opening when open, and the total flat panel width is that rod length multiplied by a fullness ratio — 1.5x for a tailored look, 2x for a normal gather, 2.5x for something opulent. Length runs from the rod down: the height the rod sits above the window, plus the window itself, plus whatever you want below the sill, plus about 8 inches for the header and hem. Hanging the rod high and wide is the single most effective trick in the trade, because it makes the window read as larger and lets the panels clear the glass entirely.
Curtain Size
Rod width = window width + 16 in; total panel width = rod width x fullness; length = rod height above window + window height + drop below sill + 8 in for header and hem
Four to six inches for a normal ceiling, or right up near the cornice if the ceiling is high. Mounting at the window frame makes the wall look shorter and the window smaller.
2x the rod width is the standard gather and looks right on almost everything. 1.5x suits heavy or patterned fabric where you want the print readable; 2.5x is for sheers and formal rooms.
Either just kiss it (about half an inch clear) or break onto it by an inch. The one length to avoid is stopping a few inches short, which reads as an error rather than a choice.