A 30-inch vanity can work in a 6 x 7.5-foot prewar bath, but the real test is not the width by itself. It is the usable path between the tub edge, the vanity corner, and the door swing once the counter overhang is included. I would mock up the full vanity depth on the floor and wall with painter's tape or cardboard before ordering anything.

If that mockup feels tight, going slightly narrower or shallower often improves the room more than people expect. In small baths, 2 or 3 inches of extra circulation usually matters more than squeezing in the biggest cabinet possible. A wall-mounted vanity can also help the room feel lighter if storage is limited.

From a production and QC perspective, I would also check the faucet reach, drawer or door opening clearance, and whether a recessed medicine cabinet can make up the storage you lose by choosing a smaller vanity. That combination often works better than forcing one oversized base cabinet into a narrow plan.

Thank you, I didn't consider the vanity drawer and door reach!

The IRC code requires a minimum 21-inch clearance in front of the vanity -- it looks like your 20-inch deep vanity just barely allows for that. I am assuming you are asking about a vanity that is 30 inches wide, and not a (trough?) sink.

A wall-mounted faucet will allow for a narrow vanity; otherwise, it may be difficult to find a narrow sink that can work with a deck-mounted faucet on a 20-inch deep vanity.

Recommend 18" vanity, then 30" ea for toilet, tub against back wall.

HomDera looks great! But I don't see a how to resize fixture items or change it to imperial units. just one inch makes a big difference. Is there a website that can help?