It will definitely be too tight.

Absolutely NOT. I’m schocked that you and your “contractor” don’t understand code requirements here.

I had a builder come to give me an estimate for a different room, and he thought it could support the 42". I think the previous owner had a vanity that size in this powder.

If your toilet is code minimum from the wall, and if the interior room dimension is truly 72", then the 42" vanity will be right at code minimum clearance for the toilet. Is that really what you want?

Thank you for your comments; would you do a 37" vanity? Or do you think that would take up too much space? It's appealing to me for the extra counter space, however the current vanity is tapered with the front at 21" and the back at 32.5." Will that be too big for that space please? If so, there is a 31" version but countertop is obviously smaller.

This is 31"

This is 37":