The industry standard for a shower curtain is 72 inches by 72 inches, a figure that can be tweaked by individual manufacturers to anywhere between 70 and 72 inches. For a stall shower, which has a taller and skinnier opening to be protected from spray than the side or perimeter of a tub, the dimensions are typically 50 inches by 78 inches, although some stalls are wider and might take up to 54 inches of fabric. Your bathroom configuration and your tub or shower will determine the ideal size for your shower curtain.
Tubs tend to be about 6 feet long and the shower curtain hangs about 6 feet from ground or inner tub to rod. But your antique clawfoot tub needs a custom shape. Those curtains can be 180 inches wide to encircle the tub, and about 68 inches tall, to hang from a rod suspended over the tub and reach slightly inside its perimeter when pulled closed. Any unusual configuration is going to require a bespoke curtain, or some careful trimming with sharp scissors.
Measure your tub/shower area for a curtain, using a metal tape measure for accuracy. For the curtain width, measure the length of the tub, the shower opening or the curtain rod, and then add a percentage for folds. Twenty percent more fabric will give you standard fullness when the curtain is pulled across the opening; 50 percent is more generous; doubling the width of the curtain is pure luxury.
In a minimalist bathroom, stick to modest fullness. Save the opulence for your mini-Versailles bathing chamber. Vertical length is measured from the bottom of the curtain rings or 2 inches below the rod -- for roller hooks -- to the floor. Subtract about 10 inches from that length for a tub and about 2 inches for a shower stall, so the curtain won't pool on the floor or the inside of the tub.